perf_event_open(2) — Linux manual page

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PERF_EVENT_OPEN(2)      Linux Programmer's Manual     PERF_EVENT_OPEN(2)

NAME         top

       perf_event_open - set up performance monitoring

SYNOPSIS         top

       #include <linux/perf_event.h>    /* Definition of PERF_* constants */
       #include <linux/hw_breakpoint.h> /* Definition of HW_* constants */
       #include <sys/syscall.h>         /* Definition of SYS_* constants */
       #include <unistd.h>

       int syscall(SYS_perf_event_open, struct perf_event_attr *attr,
                   pid_t pid, int cpu, int group_fd, unsigned long flags);

       Note: glibc provides no wrapper for perf_event_open(),
       necessitating the use of syscall(2).

DESCRIPTION         top

       Given a list of parameters, perf_event_open() returns a file
       descriptor, for use in subsequent system calls (read(2), mmap(2),
       prctl(2), fcntl(2), etc.).

       A call to perf_event_open() creates a file descriptor that allows
       measuring performance information.  Each file descriptor
       corresponds to one event that is measured; these can be grouped
       together to measure multiple events simultaneously.

       Events can be enabled and disabled in two ways: via ioctl(2) and
       via prctl(2).  When an event is disabled it does not count or
       generate overflows but does continue to exist and maintain its
       count value.

       Events come in two flavors: counting and sampled.  A counting
       event is one that is used for counting the aggregate number of
       events that occur.  In general, counting event results are
       gathered with a read(2) call.  A sampling event periodically
       writes measurements to a buffer that can then be accessed via
       mmap(2).

   Arguments
       The pid and cpu arguments allow specifying which process and CPU
       to monitor:

       pid == 0 and cpu == -1
              This measures the calling process/thread on any CPU.

       pid == 0 and cpu >= 0
              This measures the calling process/thread only when running
              on the specified CPU.

       pid > 0 and cpu == -1
              This measures the specified process/thread on any CPU.

       pid > 0 and cpu >= 0
              This measures the specified process/thread only when
              running on the specified CPU.

       pid == -1 and cpu >= 0
              This measures all processes/threads on the specified CPU.
              This requires CAP_PERFMON (since Linux 5.8) or
              CAP_SYS_ADMIN capability or a
              /proc/sys/kernel/perf_event_paranoid value of less than 1.

       pid == -1 and cpu == -1
              This setting is invalid and will return an error.

       When pid is greater than zero, permission to perform this system
       call is governed by CAP_PERFMON (since Linux 5.9) and a ptrace
       access mode PTRACE_MODE_READ_REALCREDS check on older Linux
       versions; see ptrace(2).

       The group_fd argument allows event groups to be created.  An
       event group has one event which is the group leader.  The leader
       is created first, with group_fd = -1.  The rest of the group
       members are created with subsequent perf_event_open() calls with
       group_fd being set to the file descriptor of the group leader.
       (A single event on its own is created with group_fd = -1 and is
       considered to be a group with only 1 member.)  An event group is
       scheduled onto the CPU as a unit: it will be put onto the CPU
       only if all of the events in the group can be put onto the CPU.
       This means that the values of the member events can be
       meaningfully compared—added, divided (to get ratios), and so on—
       with each other, since they have counted events for the same set
       of executed instructions.

       The flags argument is formed by ORing together zero or more of
       the following values:

       PERF_FLAG_FD_CLOEXEC (since Linux 3.14)
              This flag enables the close-on-exec flag for the created
              event file descriptor, so that the file descriptor is
              automatically closed on execve(2).  Setting the close-on-
              exec flags at creation time, rather than later with
              fcntl(2), avoids potential race conditions where the
              calling thread invokes perf_event_open() and fcntl(2) at
              the same time as another thread calls fork(2) then
              execve(2).

       PERF_FLAG_FD_NO_GROUP
              This flag tells the event to ignore the group_fd parameter
              except for the purpose of setting up output redirection
              using the PERF_FLAG_FD_OUTPUT flag.

       PERF_FLAG_FD_OUTPUT (broken since Linux 2.6.35)
              This flag re-routes the event's sampled output to instead
              be included in the mmap buffer of the event specified by
              group_fd.

       PERF_FLAG_PID_CGROUP (since Linux 2.6.39)
              This flag activates per-container system-wide monitoring.
              A container is an abstraction that isolates a set of
              resources for finer-grained control (CPUs, memory, etc.).
              In this mode, the event is measured only if the thread
              running on the monitored CPU belongs to the designated
              container (cgroup).  The cgroup is identified by passing a
              file descriptor opened on its directory in the cgroupfs
              filesystem.  For instance, if the cgroup to monitor is
              called test, then a file descriptor opened on
              /dev/cgroup/test (assuming cgroupfs is mounted on
              /dev/cgroup) must be passed as the pid parameter.  cgroup
              monitoring is available only for system-wide events and
              may therefore require extra permissions.

       The perf_event_attr structure provides detailed configuration
       information for the event being created.

           struct perf_event_attr {
               __u32 type;                 /* Type of event */
               __u32 size;                 /* Size of attribute structure */
               __u64 config;               /* Type-specific configuration */

               union {
                   __u64 sample_period;    /* Period of sampling */
                   __u64 sample_freq;      /* Frequency of sampling */
               };

               __u64 sample_type;  /* Specifies values included in sample */
               __u64 read_format;  /* Specifies values returned in read */

               __u64 disabled       : 1,   /* off by default */
                     inherit        : 1,   /* children inherit it */
                     pinned         : 1,   /* must always be on PMU */
                     exclusive      : 1,   /* only group on PMU */
                     exclude_user   : 1,   /* don't count user */
                     exclude_kernel : 1,   /* don't count kernel */
                     exclude_hv     : 1,   /* don't count hypervisor */
                     exclude_idle   : 1,   /* don't count when idle */
                     mmap           : 1,   /* include mmap data */
                     comm           : 1,   /* include comm data */
                     freq           : 1,   /* use freq, not period */
                     inherit_stat   : 1,   /* per task counts */
                     enable_on_exec : 1,   /* next exec enables */
                     task           : 1,   /* trace fork/exit */
                     watermark      : 1,   /* wakeup_watermark */
                     precise_ip     : 2,   /* skid constraint */
                     mmap_data      : 1,   /* non-exec mmap data */
                     sample_id_all  : 1,   /* sample_type all events */
                     exclude_host   : 1,   /* don't count in host */
                     exclude_guest  : 1,   /* don't count in guest */
                     exclude_callchain_kernel : 1,
                                           /* exclude kernel callchains */
                     exclude_callchain_user   : 1,
                                           /* exclude user callchains */
                     mmap2          :  1,  /* include mmap with inode data */
                     comm_exec      :  1,  /* flag comm events that are
                                              due to exec */
                     use_clockid    :  1,  /* use clockid for time fields */
                     context_switch :  1,  /* context switch data */
                     write_backward :  1,  /* Write ring buffer from end
                                              to beginning */
                     namespaces     :  1,  /* include namespaces data */
                     ksymbol        :  1,  /* include ksymbol events */
                     bpf_event      :  1,  /* include bpf events */
                     aux_output     :  1,  /* generate AUX records
                                              instead of events */
                     cgroup         :  1,  /* include cgroup events */
                     text_poke      :  1,  /* include text poke events */

                     __reserved_1   : 30;

               union {
                   __u32 wakeup_events;    /* wakeup every n events */
                   __u32 wakeup_watermark; /* bytes before wakeup */
               };

               __u32     bp_type;          /* breakpoint type */

               union {
                   __u64 bp_addr;          /* breakpoint address */
                   __u64 kprobe_func;      /* for perf_kprobe */
                   __u64 uprobe_path;      /* for perf_uprobe */
                   __u64 config1;          /* extension of config */
               };

               union {
                   __u64 bp_len;           /* breakpoint length */
                   __u64 kprobe_addr;      /* with kprobe_func == NULL */
                   __u64 probe_offset;     /* for perf_[k,u]probe */
                   __u64 config2;          /* extension of config1 */
               };
               __u64 branch_sample_type;   /* enum perf_branch_sample_type */
               __u64 sample_regs_user;     /* user regs to dump on samples */
               __u32 sample_stack_user;    /* size of stack to dump on
                                              samples */
               __s32 clockid;              /* clock to use for time fields */
               __u64 sample_regs_intr;     /* regs to dump on samples */
               __u32 aux_watermark;        /* aux bytes before wakeup */
               __u16 sample_max_stack;     /* max frames in callchain */
               __u16 __reserved_2;         /* align to u64 */

           };

       The fields of the perf_event_attr structure are described in more
       detail below:

       type   This field specifies the overall event type.  It has one
              of the following values:

              PERF_TYPE_HARDWARE
                     This indicates one of the "generalized" hardware
                     events provided by the kernel.  See the config
                     field definition for more details.

              PERF_TYPE_SOFTWARE
                     This indicates one of the software-defined events
                     provided by the kernel (even if no hardware support
                     is available).

              PERF_TYPE_TRACEPOINT
                     This indicates a tracepoint provided by the kernel
                     tracepoint infrastructure.

              PERF_TYPE_HW_CACHE
                     This indicates a hardware cache event.  This has a
                     special encoding, described in the config field
                     definition.

              PERF_TYPE_RAW
                     This indicates a "raw" implementation-specific
                     event in the config field.

              PERF_TYPE_BREAKPOINT (since Linux 2.6.33)
                     This indicates a hardware breakpoint as provided by
                     the CPU.  Breakpoints can be read/write accesses to
                     an address as well as execution of an instruction
                     address.

              dynamic PMU
                     Since Linux 2.6.38, perf_event_open() can support
                     multiple PMUs.  To enable this, a value exported by
                     the kernel can be used in the type field to
                     indicate which PMU to use.  The value to use can be
                     found in the sysfs filesystem: there is a
                     subdirectory per PMU instance under
                     /sys/bus/event_source/devices.  In each
                     subdirectory there is a type file whose content is
                     an integer that can be used in the type field.  For
                     instance, /sys/bus/event_source/devices/cpu/type
                     contains the value for the core CPU PMU, which is
                     usually 4.

              kprobe and uprobe (since Linux 4.17)
                     These two dynamic PMUs create a kprobe/uprobe and
                     attach it to the file descriptor generated by
                     perf_event_open.  The kprobe/uprobe will be
                     destroyed on the destruction of the file
                     descriptor.  See fields kprobe_func, uprobe_path,
                     kprobe_addr, and probe_offset for more details.

       size   The size of the perf_event_attr structure for
              forward/backward compatibility.  Set this using
              sizeof(struct perf_event_attr) to allow the kernel to see
              the struct size at the time of compilation.

              The related define PERF_ATTR_SIZE_VER0 is set to 64; this
              was the size of the first published struct.
              PERF_ATTR_SIZE_VER1 is 72, corresponding to the addition
              of breakpoints in Linux 2.6.33.  PERF_ATTR_SIZE_VER2 is 80
              corresponding to the addition of branch sampling in Linux
              3.4.  PERF_ATTR_SIZE_VER3 is 96 corresponding to the
              addition of sample_regs_user and sample_stack_user in
              Linux 3.7.  PERF_ATTR_SIZE_VER4 is 104 corresponding to
              the addition of sample_regs_intr in Linux 3.19.
              PERF_ATTR_SIZE_VER5 is 112 corresponding to the addition
              of aux_watermark in Linux 4.1.

       config This specifies which event you want, in conjunction with
              the type field.  The config1 and config2 fields are also
              taken into account in cases where 64 bits is not enough to
              fully specify the event.  The encoding of these fields are
              event dependent.

              There are various ways to set the config field that are
              dependent on the value of the previously described type
              field.  What follows are various possible settings for
              config separated out by type.

              If type is PERF_TYPE_HARDWARE, we are measuring one of the
              generalized hardware CPU events.  Not all of these are
              available on all platforms.  Set config to one of the
              following:

                   PERF_COUNT_HW_CPU_CYCLES
                          Total cycles.  Be wary of what happens during
                          CPU frequency scaling.

                   PERF_COUNT_HW_INSTRUCTIONS
                          Retired instructions.  Be careful, these can
                          be affected by various issues, most notably
                          hardware interrupt counts.

                   PERF_COUNT_HW_CACHE_REFERENCES
                          Cache accesses.  Usually this indicates Last
                          Level Cache accesses but this may vary
                          depending on your CPU.  This may include
                          prefetches and coherency messages; again this
                          depends on the design of your CPU.

                   PERF_COUNT_HW_CACHE_MISSES
                          Cache misses.  Usually this indicates Last
                          Level Cache misses; this is intended to be
                          used in conjunction with the
                          PERF_COUNT_HW_CACHE_REFERENCES event to
                          calculate cache miss rates.

                   PERF_COUNT_HW_BRANCH_INSTRUCTIONS
                          Retired branch instructions.  Prior to Linux
                          2.6.35, this used the wrong event on AMD
                          processors.

                   PERF_COUNT_HW_BRANCH_MISSES
                          Mispredicted branch instructions.

                   PERF_COUNT_HW_BUS_CYCLES
                          Bus cycles, which can be different from total
                          cycles.

                   PERF_COUNT_HW_STALLED_CYCLES_FRONTEND (since Linux
                   3.0)
                          Stalled cycles during issue.

                   PERF_COUNT_HW_STALLED_CYCLES_BACKEND (since Linux
                   3.0)
                          Stalled cycles during retirement.

                   PERF_COUNT_HW_REF_CPU_CYCLES (since Linux 3.3)
                          Total cycles; not affected by CPU frequency
                          scaling.

              If type is PERF_TYPE_SOFTWARE, we are measuring software
              events provided by the kernel.  Set config to one of the
              following:

                   PERF_COUNT_SW_CPU_CLOCK
                          This reports the CPU clock, a high-resolution
                          per-CPU timer.

                   PERF_COUNT_SW_TASK_CLOCK
                          This reports a clock count specific to the
                          task that is running.

                   PERF_COUNT_SW_PAGE_FAULTS
                          This reports the number of page faults.

                   PERF_COUNT_SW_CONTEXT_SWITCHES
                          This counts context switches.  Until Linux
                          2.6.34, these were all reported as user-space
                          events, after that they are reported as
                          happening in the kernel.

                   PERF_COUNT_SW_CPU_MIGRATIONS
                          This reports the number of times the process
                          has migrated to a new CPU.

                   PERF_COUNT_SW_PAGE_FAULTS_MIN
                          This counts the number of minor page faults.
                          These did not require disk I/O to handle.

                   PERF_COUNT_SW_PAGE_FAULTS_MAJ
                          This counts the number of major page faults.
                          These required disk I/O to handle.

                   PERF_COUNT_SW_ALIGNMENT_FAULTS (since Linux 2.6.33)
                          This counts the number of alignment faults.
                          These happen when unaligned memory accesses
                          happen; the kernel can handle these but it
                          reduces performance.  This happens only on
                          some architectures (never on x86).

                   PERF_COUNT_SW_EMULATION_FAULTS (since Linux 2.6.33)
                          This counts the number of emulation faults.
                          The kernel sometimes traps on unimplemented
                          instructions and emulates them for user space.
                          This can negatively impact performance.

                   PERF_COUNT_SW_DUMMY (since Linux 3.12)
                          This is a placeholder event that counts
                          nothing.  Informational sample record types
                          such as mmap or comm must be associated with
                          an active event.  This dummy event allows
                          gathering such records without requiring a
                          counting event.

              If type is PERF_TYPE_TRACEPOINT, then we are measuring
              kernel tracepoints.  The value to use in config can be
              obtained from under debugfs tracing/events/*/*/id if
              ftrace is enabled in the kernel.

              If type is PERF_TYPE_HW_CACHE, then we are measuring a
              hardware CPU cache event.  To calculate the appropriate
              config value, use the following equation:

                      config = (perf_hw_cache_id) |
                               (perf_hw_cache_op_id << 8) |
                               (perf_hw_cache_op_result_id << 16);

                  where perf_hw_cache_id is one of:

                      PERF_COUNT_HW_CACHE_L1D
                             for measuring Level 1 Data Cache

                      PERF_COUNT_HW_CACHE_L1I
                             for measuring Level 1 Instruction Cache

                      PERF_COUNT_HW_CACHE_LL
                             for measuring Last-Level Cache

                      PERF_COUNT_HW_CACHE_DTLB
                             for measuring the Data TLB

                      PERF_COUNT_HW_CACHE_ITLB
                             for measuring the Instruction TLB

                      PERF_COUNT_HW_CACHE_BPU
                             for measuring the branch prediction unit

                      PERF_COUNT_HW_CACHE_NODE (since Linux 3.1)
                             for measuring local memory accesses

                  and perf_hw_cache_op_id is one of:

                      PERF_COUNT_HW_CACHE_OP_READ
                             for read accesses

                      PERF_COUNT_HW_CACHE_OP_WRITE
                             for write accesses

                      PERF_COUNT_HW_CACHE_OP_PREFETCH
                             for prefetch accesses

                  and perf_hw_cache_op_result_id is one of:

                      PERF_COUNT_HW_CACHE_RESULT_ACCESS
                             to measure accesses

                      PERF_COUNT_HW_CACHE_RESULT_MISS
                             to measure misses

              If type is PERF_TYPE_RAW, then a custom "raw" config value
              is needed.  Most CPUs support events that are not covered
              by the "generalized" events.  These are implementation
              defined; see your CPU manual (for example the Intel Volume
              3B documentation or the AMD BIOS and Kernel Developer
              Guide).  The libpfm4 library can be used to translate from
              the name in the architectural manuals to the raw hex value
              perf_event_open() expects in this field.

              If type is PERF_TYPE_BREAKPOINT, then leave config set to
              zero.  Its parameters are set in other places.

              If type is kprobe or uprobe, set retprobe (bit 0 of
              config, see
              /sys/bus/event_source/devices/[k,u]probe/format/retprobe)
              for kretprobe/uretprobe.  See fields kprobe_func,
              uprobe_path, kprobe_addr, and probe_offset for more
              details.

       kprobe_func, uprobe_path, kprobe_addr, and probe_offset
              These fields describe the kprobe/uprobe for dynamic PMUs
              kprobe and uprobe.  For kprobe: use kprobe_func and
              probe_offset, or use kprobe_addr and leave kprobe_func as
              NULL.  For uprobe: use uprobe_path and probe_offset.

       sample_period, sample_freq
              A "sampling" event is one that generates an overflow
              notification every N events, where N is given by
              sample_period.  A sampling event has sample_period > 0.
              When an overflow occurs, requested data is recorded in the
              mmap buffer.  The sample_type field controls what data is
              recorded on each overflow.

              sample_freq can be used if you wish to use frequency
              rather than period.  In this case, you set the freq flag.
              The kernel will adjust the sampling period to try and
              achieve the desired rate.  The rate of adjustment is a
              timer tick.

       sample_type
              The various bits in this field specify which values to
              include in the sample.  They will be recorded in a ring-
              buffer, which is available to user space using mmap(2).
              The order in which the values are saved in the sample are
              documented in the MMAP Layout subsection below; it is not
              the enum perf_event_sample_format order.

              PERF_SAMPLE_IP
                     Records instruction pointer.

              PERF_SAMPLE_TID
                     Records the process and thread IDs.

              PERF_SAMPLE_TIME
                     Records a timestamp.

              PERF_SAMPLE_ADDR
                     Records an address, if applicable.

              PERF_SAMPLE_READ
                     Record counter values for all events in a group,
                     not just the group leader.

              PERF_SAMPLE_CALLCHAIN
                     Records the callchain (stack backtrace).

              PERF_SAMPLE_ID
                     Records a unique ID for the opened event's group
                     leader.

              PERF_SAMPLE_CPU
                     Records CPU number.

              PERF_SAMPLE_PERIOD
                     Records the current sampling period.

              PERF_SAMPLE_STREAM_ID
                     Records a unique ID for the opened event.  Unlike
                     PERF_SAMPLE_ID the actual ID is returned, not the
                     group leader.  This ID is the same as the one
                     returned by PERF_FORMAT_ID.

              PERF_SAMPLE_RAW
                     Records additional data, if applicable.  Usually
                     returned by tracepoint events.

              PERF_SAMPLE_BRANCH_STACK (since Linux 3.4)
                     This provides a record of recent branches, as
                     provided by CPU branch sampling hardware (such as
                     Intel Last Branch Record).  Not all hardware
                     supports this feature.

                     See the branch_sample_type field for how to filter
                     which branches are reported.

              PERF_SAMPLE_REGS_USER (since Linux 3.7)
                     Records the current user-level CPU register state
                     (the values in the process before the kernel was
                     called).

              PERF_SAMPLE_STACK_USER (since Linux 3.7)
                     Records the user level stack, allowing stack
                     unwinding.

              PERF_SAMPLE_WEIGHT (since Linux 3.10)
                     Records a hardware provided weight value that
                     expresses how costly the sampled event was.  This
                     allows the hardware to highlight expensive events
                     in a profile.

              PERF_SAMPLE_DATA_SRC (since Linux 3.10)
                     Records the data source: where in the memory
                     hierarchy the data associated with the sampled
                     instruction came from.  This is available only if
                     the underlying hardware supports this feature.

              PERF_SAMPLE_IDENTIFIER (since Linux 3.12)
                     Places the SAMPLE_ID value in a fixed position in
                     the record, either at the beginning (for sample
                     events) or at the end (if a non-sample event).

                     This was necessary because a sample stream may have
                     records from various different event sources with
                     different sample_type settings.  Parsing the event
                     stream properly was not possible because the format
                     of the record was needed to find SAMPLE_ID, but the
                     format could not be found without knowing what
                     event the sample belonged to (causing a circular
                     dependency).

                     The PERF_SAMPLE_IDENTIFIER setting makes the event
                     stream always parsable by putting SAMPLE_ID in a
                     fixed location, even though it means having
                     duplicate SAMPLE_ID values in records.

              PERF_SAMPLE_TRANSACTION (since Linux 3.13)
                     Records reasons for transactional memory abort
                     events (for example, from Intel TSX transactional
                     memory support).

                     The precise_ip setting must be greater than 0 and a
                     transactional memory abort event must be measured
                     or no values will be recorded.  Also note that some
                     perf_event measurements, such as sampled cycle
                     counting, may cause extraneous aborts (by causing
                     an interrupt during a transaction).

              PERF_SAMPLE_REGS_INTR (since Linux 3.19)
                     Records a subset of the current CPU register state
                     as specified by sample_regs_intr.  Unlike
                     PERF_SAMPLE_REGS_USER the register values will
                     return kernel register state if the overflow
                     happened while kernel code is running.  If the CPU
                     supports hardware sampling of register state (i.e.,
                     PEBS on Intel x86) and precise_ip is set higher
                     than zero then the register values returned are
                     those captured by hardware at the time of the
                     sampled instruction's retirement.

              PERF_SAMPLE_PHYS_ADDR (since Linux 4.13)
                     Records physical address of data like in
                     PERF_SAMPLE_ADDR.

              PERF_SAMPLE_CGROUP (since Linux 5.7)
                     Records (perf_event) cgroup ID of the process.
                     This corresponds to the id field in the
                     PERF_RECORD_CGROUP event.

       read_format
              This field specifies the format of the data returned by
              read(2) on a perf_event_open() file descriptor.

              PERF_FORMAT_TOTAL_TIME_ENABLED
                     Adds the 64-bit time_enabled field.  This can be
                     used to calculate estimated totals if the PMU is
                     overcommitted and multiplexing is happening.

              PERF_FORMAT_TOTAL_TIME_RUNNING
                     Adds the 64-bit time_running field.  This can be
                     used to calculate estimated totals if the PMU is
                     overcommitted and multiplexing is happening.

              PERF_FORMAT_ID
                     Adds a 64-bit unique value that corresponds to the
                     event group.

              PERF_FORMAT_GROUP
                     Allows all counter values in an event group to be
                     read with one read.

       disabled
              The disabled bit specifies whether the counter starts out
              disabled or enabled.  If disabled, the event can later be
              enabled by ioctl(2), prctl(2), or enable_on_exec.

              When creating an event group, typically the group leader
              is initialized with disabled set to 1 and any child events
              are initialized with disabled set to 0.  Despite disabled
              being 0, the child events will not start until the group
              leader is enabled.

       inherit
              The inherit bit specifies that this counter should count
              events of child tasks as well as the task specified.  This
              applies only to new children, not to any existing children
              at the time the counter is created (nor to any new
              children of existing children).

              Inherit does not work for some combinations of read_format
              values, such as PERF_FORMAT_GROUP.

       pinned The pinned bit specifies that the counter should always be
              on the CPU if at all possible.  It applies only to
              hardware counters and only to group leaders.  If a pinned
              counter cannot be put onto the CPU (e.g., because there
              are not enough hardware counters or because of a conflict
              with some other event), then the counter goes into an
              'error' state, where reads return end-of-file (i.e.,
              read(2) returns 0) until the counter is subsequently
              enabled or disabled.

       exclusive
              The exclusive bit specifies that when this counter's group
              is on the CPU, it should be the only group using the CPU's
              counters.  In the future this may allow monitoring
              programs to support PMU features that need to run alone so
              that they do not disrupt other hardware counters.

              Note that many unexpected situations may prevent events
              with the exclusive bit set from ever running.  This
              includes any users running a system-wide measurement as
              well as any kernel use of the performance counters
              (including the commonly enabled NMI Watchdog Timer
              interface).

       exclude_user
              If this bit is set, the count excludes events that happen
              in user space.

       exclude_kernel
              If this bit is set, the count excludes events that happen
              in kernel space.

       exclude_hv
              If this bit is set, the count excludes events that happen
              in the hypervisor.  This is mainly for PMUs that have
              built-in support for handling this (such as POWER).  Extra
              support is needed for handling hypervisor measurements on
              most machines.

       exclude_idle
              If set, don't count when the CPU is running the idle task.
              While you can currently enable this for any event type, it
              is ignored for all but software events.

       mmap   The mmap bit enables generation of PERF_RECORD_MMAP
              samples for every mmap(2) call that has PROT_EXEC set.
              This allows tools to notice new executable code being
              mapped into a program (dynamic shared libraries for
              example) so that addresses can be mapped back to the
              original code.

       comm   The comm bit enables tracking of process command name as
              modified by the execve(2) and prctl(PR_SET_NAME) system
              calls as well as writing to /proc/self/comm.  If the
              comm_exec flag is also successfully set (possible since
              Linux 3.16), then the misc flag PERF_RECORD_MISC_COMM_EXEC
              can be used to differentiate the execve(2) case from the
              others.

       freq   If this bit is set, then sample_frequency not
              sample_period is used when setting up the sampling
              interval.

       inherit_stat
              This bit enables saving of event counts on context switch
              for inherited tasks.  This is meaningful only if the
              inherit field is set.

       enable_on_exec
              If this bit is set, a counter is automatically enabled
              after a call to execve(2).

       task   If this bit is set, then fork/exit notifications are
              included in the ring buffer.

       watermark
              If set, have an overflow notification happen when we cross
              the wakeup_watermark boundary.  Otherwise, overflow
              notifications happen after wakeup_events samples.

       precise_ip (since Linux 2.6.35)
              This controls the amount of skid.  Skid is how many
              instructions execute between an event of interest
              happening and the kernel being able to stop and record the
              event.  Smaller skid is better and allows more accurate
              reporting of which events correspond to which
              instructions, but hardware is often limited with how small
              this can be.

              The possible values of this field are the following:

              0  SAMPLE_IP can have arbitrary skid.

              1  SAMPLE_IP must have constant skid.

              2  SAMPLE_IP requested to have 0 skid.

              3  SAMPLE_IP must have 0 skid.  See also the description
                 of PERF_RECORD_MISC_EXACT_IP.

       mmap_data (since Linux 2.6.36)
              This is the counterpart of the mmap field.  This enables
              generation of PERF_RECORD_MMAP samples for mmap(2) calls
              that do not have PROT_EXEC set (for example data and SysV
              shared memory).

       sample_id_all (since Linux 2.6.38)
              If set, then TID, TIME, ID, STREAM_ID, and CPU can
              additionally be included in non-PERF_RECORD_SAMPLEs if the
              corresponding sample_type is selected.

              If PERF_SAMPLE_IDENTIFIER is specified, then an additional
              ID value is included as the last value to ease parsing the
              record stream.  This may lead to the id value appearing
              twice.

              The layout is described by this pseudo-structure:

                  struct sample_id {
                      { u32 pid, tid; }   /* if PERF_SAMPLE_TID set */
                      { u64 time;     }   /* if PERF_SAMPLE_TIME set */
                      { u64 id;       }   /* if PERF_SAMPLE_ID set */
                      { u64 stream_id;}   /* if PERF_SAMPLE_STREAM_ID set  */
                      { u32 cpu, res; }   /* if PERF_SAMPLE_CPU set */
                      { u64 id;       }   /* if PERF_SAMPLE_IDENTIFIER set */
                  };

       exclude_host (since Linux 3.2)
              When conducting measurements that include processes
              running VM instances (i.e., have executed a KVM_RUN
              ioctl(2)), only measure events happening inside a guest
              instance.  This is only meaningful outside the guests;
              this setting does not change counts gathered inside of a
              guest.  Currently, this functionality is x86 only.

       exclude_guest (since Linux 3.2)
              When conducting measurements that include processes
              running VM instances (i.e., have executed a KVM_RUN
              ioctl(2)), do not measure events happening inside guest
              instances.  This is only meaningful outside the guests;
              this setting does not change counts gathered inside of a
              guest.  Currently, this functionality is x86 only.

       exclude_callchain_kernel (since Linux 3.7)
              Do not include kernel callchains.

       exclude_callchain_user (since Linux 3.7)
              Do not include user callchains.

       mmap2 (since Linux 3.16)
              Generate an extended executable mmap record that contains
              enough additional information to uniquely identify shared
              mappings.  The mmap flag must also be set for this to
              work.

       comm_exec (since Linux 3.16)
              This is purely a feature-detection flag, it does not
              change kernel behavior.  If this flag can successfully be
              set, then, when comm is enabled, the
              PERF_RECORD_MISC_COMM_EXEC flag will be set in the misc
              field of a comm record header if the rename event being
              reported was caused by a call to execve(2).  This allows
              tools to distinguish between the various types of process
              renaming.

       use_clockid (since Linux 4.1)
              This allows selecting which internal Linux clock to use
              when generating timestamps via the clockid field.  This
              can make it easier to correlate perf sample times with
              timestamps generated by other tools.

       context_switch (since Linux 4.3)
              This enables the generation of PERF_RECORD_SWITCH records
              when a context switch occurs.  It also enables the
              generation of PERF_RECORD_SWITCH_CPU_WIDE records when
              sampling in CPU-wide mode.  This functionality is in
              addition to existing tracepoint and software events for
              measuring context switches.  The advantage of this method
              is that it will give full information even with strict
              perf_event_paranoid settings.

       write_backward (since Linux 4.6)
              This causes the ring buffer to be written from the end to
              the beginning.  This is to support reading from
              overwritable ring buffer.

       namespaces (since Linux 4.11)
              This enables the generation of PERF_RECORD_NAMESPACES
              records when a task enters a new namespace.  Each
              namespace has a combination of device and inode numbers.

       ksymbol (since Linux 5.0)
              This enables the generation of PERF_RECORD_KSYMBOL records
              when new kernel symbols are registered or unregistered.
              This is analyzing dynamic kernel functions like eBPF.

       bpf_event (since Linux 5.0)
              This enables the generation of PERF_RECORD_BPF_EVENT
              records when an eBPF program is loaded or unloaded.

       auxevent (since Linux 5.4)
              This allows normal (non-AUX) events to generate data for
              AUX events if the hardware supports it.

       cgroup (since Linux 5.7)
              This enables the generation of PERF_RECORD_CGROUP records
              when a new cgroup is created (and activated).

       text_poke (since Linux 5.8)
              This enables the generation of PERF_RECORD_TEXT_POKE
              records when there's a changes to the kernel text (i.e.,
              self-modifying code).

       wakeup_events, wakeup_watermark
              This union sets how many samples (wakeup_events) or bytes
              (wakeup_watermark) happen before an overflow notification
              happens.  Which one is used is selected by the watermark
              bit flag.

              wakeup_events counts only PERF_RECORD_SAMPLE record types.
              To receive overflow notification for all PERF_RECORD types
              choose watermark and set wakeup_watermark to 1.

              Prior to Linux 3.0, setting wakeup_events to 0 resulted in
              no overflow notifications; more recent kernels treat 0 the
              same as 1.

       bp_type (since Linux 2.6.33)
              This chooses the breakpoint type.  It is one of:

              HW_BREAKPOINT_EMPTY
                     No breakpoint.

              HW_BREAKPOINT_R
                     Count when we read the memory location.

              HW_BREAKPOINT_W
                     Count when we write the memory location.

              HW_BREAKPOINT_RW
                     Count when we read or write the memory location.

              HW_BREAKPOINT_X
                     Count when we execute code at the memory location.

              The values can be combined via a bitwise or, but the
              combination of HW_BREAKPOINT_R or HW_BREAKPOINT_W with
              HW_BREAKPOINT_X is not allowed.

       bp_addr (since Linux 2.6.33)
              This is the address of the breakpoint.  For execution
              breakpoints, this is the memory address of the instruction
              of interest; for read and write breakpoints, it is the
              memory address of the memory location of interest.

       config1 (since Linux 2.6.39)
              config1 is used for setting events that need an extra
              register or otherwise do not fit in the regular config
              field.  Raw OFFCORE_EVENTS on Nehalem/Westmere/SandyBridge
              use this field on Linux 3.3 and later kernels.

       bp_len (since Linux 2.6.33)
              bp_len is the length of the breakpoint being measured if
              type is PERF_TYPE_BREAKPOINT.  Options are
              HW_BREAKPOINT_LEN_1, HW_BREAKPOINT_LEN_2,
              HW_BREAKPOINT_LEN_4, and HW_BREAKPOINT_LEN_8.  For an
              execution breakpoint, set this to sizeof(long).

       config2 (since Linux 2.6.39)
              config2 is a further extension of the config1 field.

       branch_sample_type (since Linux 3.4)
              If PERF_SAMPLE_BRANCH_STACK is enabled, then this
              specifies what branches to include in the branch record.

              The first part of the value is the privilege level, which
              is a combination of one of the values listed below.  If
              the user does not set privilege level explicitly, the
              kernel will use the event's privilege level.  Event and
              branch privilege levels do not have to match.

              PERF_SAMPLE_BRANCH_USER
                     Branch target is in user space.

              PERF_SAMPLE_BRANCH_KERNEL
                     Branch target is in kernel space.

              PERF_SAMPLE_BRANCH_HV
                     Branch target is in hypervisor.

              PERF_SAMPLE_BRANCH_PLM_ALL
                     A convenience value that is the three preceding
                     values ORed together.

              In addition to the privilege value, at least one or more
              of the following bits must be set.

              PERF_SAMPLE_BRANCH_ANY
                     Any branch type.

              PERF_SAMPLE_BRANCH_ANY_CALL
                     Any call branch (includes direct calls, indirect
                     calls, and far jumps).

              PERF_SAMPLE_BRANCH_IND_CALL
                     Indirect calls.

              PERF_SAMPLE_BRANCH_CALL (since Linux 4.4)
                     Direct calls.

              PERF_SAMPLE_BRANCH_ANY_RETURN
                     Any return branch.

              PERF_SAMPLE_BRANCH_IND_JUMP (since Linux 4.2)
                     Indirect jumps.

              PERF_SAMPLE_BRANCH_COND (since Linux 3.16)
                     Conditional branches.

              PERF_SAMPLE_BRANCH_ABORT_TX (since Linux 3.11)
                     Transactional memory aborts.

              PERF_SAMPLE_BRANCH_IN_TX (since Linux 3.11)
                     Branch in transactional memory transaction.

              PERF_SAMPLE_BRANCH_NO_TX (since Linux 3.11)
                     Branch not in transactional memory transaction.
                     PERF_SAMPLE_BRANCH_CALL_STACK (since Linux 4.1)
                     Branch is part of a hardware-generated call stack.
                     This requires hardware support, currently only
                     found on Intel x86 Haswell or newer.

       sample_regs_user (since Linux 3.7)
              This bit mask defines the set of user CPU registers to
              dump on samples.  The layout of the register mask is
              architecture-specific and is described in the kernel
              header file arch/ARCH/include/uapi/asm/perf_regs.h.

       sample_stack_user (since Linux 3.7)
              This defines the size of the user stack to dump if
              PERF_SAMPLE_STACK_USER is specified.

       clockid (since Linux 4.1)
              If use_clockid is set, then this field selects which
              internal Linux timer to use for timestamps.  The available
              timers are defined in linux/time.h, with CLOCK_MONOTONIC,
              CLOCK_MONOTONIC_RAW, CLOCK_REALTIME, CLOCK_BOOTTIME, and
              CLOCK_TAI currently supported.

       aux_watermark (since Linux 4.1)
              This specifies how much data is required to trigger a
              PERF_RECORD_AUX sample.

       sample_max_stack (since Linux 4.8)
              When sample_type includes PERF_SAMPLE_CALLCHAIN, this
              field specifies how many stack frames to report when
              generating the callchain.

   Reading results
       Once a perf_event_open() file descriptor has been opened, the
       values of the events can be read from the file descriptor.  The
       values that are there are specified by the read_format field in
       the attr structure at open time.

       If you attempt to read into a buffer that is not big enough to
       hold the data, the error ENOSPC results.

       Here is the layout of the data returned by a read:

       * If PERF_FORMAT_GROUP was specified to allow reading all events
         in a group at once:

             struct read_format {
                 u64 nr;            /* The number of events */
                 u64 time_enabled;  /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
                 u64 time_running;  /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
                 struct {
                     u64 value;     /* The value of the event */
                     u64 id;        /* if PERF_FORMAT_ID */
                 } values[nr];
             };

       * If PERF_FORMAT_GROUP was not specified:

             struct read_format {
                 u64 value;         /* The value of the event */
                 u64 time_enabled;  /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
                 u64 time_running;  /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
                 u64 id;            /* if PERF_FORMAT_ID */
             };

       The values read are as follows:

       nr     The number of events in this file descriptor.  Available
              only if PERF_FORMAT_GROUP was specified.

       time_enabled, time_running
              Total time the event was enabled and running.  Normally
              these values are the same.  Multiplexing happens if the
              number of events is more than the number of available PMU
              counter slots.  In that case the events run only part of
              the time and the time_enabled and time running values can
              be used to scale an estimated value for the count.

       value  An unsigned 64-bit value containing the counter result.

       id     A globally unique value for this particular event; only
              present if PERF_FORMAT_ID was specified in read_format.

   MMAP layout
       When using perf_event_open() in sampled mode, asynchronous events
       (like counter overflow or PROT_EXEC mmap tracking) are logged
       into a ring-buffer.  This ring-buffer is created and accessed
       through mmap(2).

       The mmap size should be 1+2^n pages, where the first page is a
       metadata page (struct perf_event_mmap_page) that contains various
       bits of information such as where the ring-buffer head is.

       Before kernel 2.6.39, there is a bug that means you must allocate
       an mmap ring buffer when sampling even if you do not plan to
       access it.

       The structure of the first metadata mmap page is as follows:

           struct perf_event_mmap_page {
               __u32 version;        /* version number of this structure */
               __u32 compat_version; /* lowest version this is compat with */
               __u32 lock;           /* seqlock for synchronization */
               __u32 index;          /* hardware counter identifier */
               __s64 offset;         /* add to hardware counter value */
               __u64 time_enabled;   /* time event active */
               __u64 time_running;   /* time event on CPU */
               union {
                   __u64   capabilities;
                   struct {
                       __u64 cap_usr_time / cap_usr_rdpmc / cap_bit0 : 1,
                             cap_bit0_is_deprecated : 1,
                             cap_user_rdpmc         : 1,
                             cap_user_time          : 1,
                             cap_user_time_zero     : 1,
                   };
               };
               __u16 pmc_width;
               __u16 time_shift;
               __u32 time_mult;
               __u64 time_offset;
               __u64 __reserved[120];   /* Pad to 1 k */
               __u64 data_head;         /* head in the data section */
               __u64 data_tail;         /* user-space written tail */
               __u64 data_offset;       /* where the buffer starts */
               __u64 data_size;         /* data buffer size */
               __u64 aux_head;
               __u64 aux_tail;
               __u64 aux_offset;
               __u64 aux_size;

           }

       The following list describes the fields in the
       perf_event_mmap_page structure in more detail:

       version
              Version number of this structure.

       compat_version
              The lowest version this is compatible with.

       lock   A seqlock for synchronization.

       index  A unique hardware counter identifier.

       offset When using rdpmc for reads this offset value must be added
              to the one returned by rdpmc to get the current total
              event count.

       time_enabled
              Time the event was active.

       time_running
              Time the event was running.

       cap_usr_time / cap_usr_rdpmc / cap_bit0 (since Linux 3.4)
              There was a bug in the definition of cap_usr_time and
              cap_usr_rdpmc from Linux 3.4 until Linux 3.11.  Both bits
              were defined to point to the same location, so it was
              impossible to know if cap_usr_time or cap_usr_rdpmc were
              actually set.

              Starting with Linux 3.12, these are renamed to cap_bit0
              and you should use the cap_user_time and cap_user_rdpmc
              fields instead.

       cap_bit0_is_deprecated (since Linux 3.12)
              If set, this bit indicates that the kernel supports the
              properly separated cap_user_time and cap_user_rdpmc bits.

              If not-set, it indicates an older kernel where
              cap_usr_time and cap_usr_rdpmc map to the same bit and
              thus both features should be used with caution.

       cap_user_rdpmc (since Linux 3.12)
              If the hardware supports user-space read of performance
              counters without syscall (this is the "rdpmc" instruction
              on x86), then the following code can be used to do a read:

                  u32 seq, time_mult, time_shift, idx, width;
                  u64 count, enabled, running;
                  u64 cyc, time_offset;

                  do {
                      seq = pc->lock;
                      barrier();
                      enabled = pc->time_enabled;
                      running = pc->time_running;

                      if (pc->cap_usr_time && enabled != running) {
                          cyc = rdtsc();
                          time_offset = pc->time_offset;
                          time_mult   = pc->time_mult;
                          time_shift  = pc->time_shift;
                      }

                      idx = pc->index;
                      count = pc->offset;

                      if (pc->cap_usr_rdpmc && idx) {
                          width = pc->pmc_width;
                          count += rdpmc(idx - 1);
                      }

                      barrier();
                  } while (pc->lock != seq);

       cap_user_time (since Linux 3.12)
              This bit indicates the hardware has a constant, nonstop
              timestamp counter (TSC on x86).

       cap_user_time_zero (since Linux 3.12)
              Indicates the presence of time_zero which allows mapping
              timestamp values to the hardware clock.

       pmc_width
              If cap_usr_rdpmc, this field provides the bit-width of the
              value read using the rdpmc or equivalent instruction.
              This can be used to sign extend the result like:

                  pmc <<= 64 - pmc_width;
                  pmc >>= 64 - pmc_width; // signed shift right
                  count += pmc;

       time_shift, time_mult, time_offset

              If cap_usr_time, these fields can be used to compute the
              time delta since time_enabled (in nanoseconds) using rdtsc
              or similar.

                  u64 quot, rem;
                  u64 delta;

                  quot  = cyc >> time_shift;
                  rem   = cyc & (((u64)1 << time_shift) - 1);
                  delta = time_offset + quot * time_mult +
                          ((rem * time_mult) >> time_shift);

              Where time_offset, time_mult, time_shift, and cyc are read
              in the seqcount loop described above.  This delta can then
              be added to enabled and possible running (if idx),
              improving the scaling:

                  enabled += delta;
                  if (idx)
                      running += delta;
                  quot  = count / running;
                  rem   = count % running;
                  count = quot * enabled + (rem * enabled) / running;

       time_zero (since Linux 3.12)

              If cap_usr_time_zero is set, then the hardware clock (the
              TSC timestamp counter on x86) can be calculated from the
              time_zero, time_mult, and time_shift values:

                  time = timestamp - time_zero;
                  quot = time / time_mult;
                  rem  = time % time_mult;
                  cyc  = (quot << time_shift) + (rem << time_shift) / time_mult;

              And vice versa:

                  quot = cyc >> time_shift;
                  rem  = cyc & (((u64)1 << time_shift) - 1);
                  timestamp = time_zero + quot * time_mult +
                              ((rem * time_mult) >> time_shift);

       data_head
              This points to the head of the data section.  The value
              continuously increases, it does not wrap.  The value needs
              to be manually wrapped by the size of the mmap buffer
              before accessing the samples.

              On SMP-capable platforms, after reading the data_head
              value, user space should issue an rmb().

       data_tail
              When the mapping is PROT_WRITE, the data_tail value should
              be written by user space to reflect the last read data.
              In this case, the kernel will not overwrite unread data.

       data_offset (since Linux 4.1)
              Contains the offset of the location in the mmap buffer
              where perf sample data begins.

       data_size (since Linux 4.1)
              Contains the size of the perf sample region within the
              mmap buffer.

       aux_head, aux_tail, aux_offset, aux_size (since Linux 4.1)
              The AUX region allows mmap(2)-ing a separate sample buffer
              for high-bandwidth data streams (separate from the main
              perf sample buffer).  An example of a high-bandwidth
              stream is instruction tracing support, as is found in
              newer Intel processors.

              To set up an AUX area, first aux_offset needs to be set
              with an offset greater than data_offset+data_size and
              aux_size needs to be set to the desired buffer size.  The
              desired offset and size must be page aligned, and the size
              must be a power of two.  These values are then passed to
              mmap in order to map the AUX buffer.  Pages in the AUX
              buffer are included as part of the RLIMIT_MEMLOCK resource
              limit (see setrlimit(2)), and also as part of the
              perf_event_mlock_kb allowance.

              By default, the AUX buffer will be truncated if it will
              not fit in the available space in the ring buffer.  If the
              AUX buffer is mapped as a read only buffer, then it will
              operate in ring buffer mode where old data will be
              overwritten by new.  In overwrite mode, it might not be
              possible to infer where the new data began, and it is the
              consumer's job to disable measurement while reading to
              avoid possible data races.

              The aux_head and aux_tail ring buffer pointers have the
              same behavior and ordering rules as the previous described
              data_head and data_tail.

       The following 2^n ring-buffer pages have the layout described
       below.

       If perf_event_attr.sample_id_all is set, then all event types
       will have the sample_type selected fields related to where/when
       (identity) an event took place (TID, TIME, ID, CPU, STREAM_ID)
       described in PERF_RECORD_SAMPLE below, it will be stashed just
       after the perf_event_header and the fields already present for
       the existing fields, that is, at the end of the payload.  This
       allows a newer perf.data file to be supported by older perf
       tools, with the new optional fields being ignored.

       The mmap values start with a header:

           struct perf_event_header {
               __u32   type;
               __u16   misc;
               __u16   size;
           };

       Below, we describe the perf_event_header fields in more detail.
       For ease of reading, the fields with shorter descriptions are
       presented first.

       size   This indicates the size of the record.

       misc   The misc field contains additional information about the
              sample.

              The CPU mode can be determined from this value by masking
              with PERF_RECORD_MISC_CPUMODE_MASK and looking for one of
              the following (note these are not bit masks, only one can
              be set at a time):

              PERF_RECORD_MISC_CPUMODE_UNKNOWN
                     Unknown CPU mode.

              PERF_RECORD_MISC_KERNEL
                     Sample happened in the kernel.

              PERF_RECORD_MISC_USER
                     Sample happened in user code.

              PERF_RECORD_MISC_HYPERVISOR
                     Sample happened in the hypervisor.

              PERF_RECORD_MISC_GUEST_KERNEL (since Linux 2.6.35)
                     Sample happened in the guest kernel.

              PERF_RECORD_MISC_GUEST_USER  (since Linux 2.6.35)
                     Sample happened in guest user code.

              Since the following three statuses are generated by
              different record types, they alias to the same bit:

              PERF_RECORD_MISC_MMAP_DATA (since Linux 3.10)
                     This is set when the mapping is not executable;
                     otherwise the mapping is executable.

              PERF_RECORD_MISC_COMM_EXEC (since Linux 3.16)
                     This is set for a PERF_RECORD_COMM record on
                     kernels more recent than Linux 3.16 if a process
                     name change was caused by an execve(2) system call.

              PERF_RECORD_MISC_SWITCH_OUT (since Linux 4.3)
                     When a PERF_RECORD_SWITCH or
                     PERF_RECORD_SWITCH_CPU_WIDE record is generated,
                     this bit indicates that the context switch is away
                     from the current process (instead of into the
                     current process).

              In addition, the following bits can be set:

              PERF_RECORD_MISC_EXACT_IP
                     This indicates that the content of PERF_SAMPLE_IP
                     points to the actual instruction that triggered the
                     event.  See also perf_event_attr.precise_ip.

              PERF_RECORD_MISC_EXT_RESERVED (since Linux 2.6.35)
                     This indicates there is extended data available
                     (currently not used).

              PERF_RECORD_MISC_PROC_MAP_PARSE_TIMEOUT
                     This bit is not set by the kernel.  It is reserved
                     for the user-space perf utility to indicate that
                     /proc/i[pid]/maps parsing was taking too long and
                     was stopped, and thus the mmap records may be
                     truncated.

       type   The type value is one of the below.  The values in the
              corresponding record (that follows the header) depend on
              the type selected as shown.

              PERF_RECORD_MMAP
                  The MMAP events record the PROT_EXEC mappings so that
                  we can correlate user-space IPs to code.  They have
                  the following structure:

                      struct {
                          struct perf_event_header header;
                          u32    pid, tid;
                          u64    addr;
                          u64    len;
                          u64    pgoff;
                          char   filename[];
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  addr   is the address of the allocated memory.  len is
                         the length of the allocated memory.  pgoff is
                         the page offset of the allocated memory.
                         filename is a string describing the backing of
                         the allocated memory.

              PERF_RECORD_LOST
                  This record indicates when events are lost.

                      struct {
                          struct perf_event_header header;
                          u64    id;
                          u64    lost;
                          struct sample_id sample_id;
                      };

                  id     is the unique event ID for the samples that
                         were lost.

                  lost   is the number of events that were lost.

              PERF_RECORD_COMM
                  This record indicates a change in the process name.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                          char   comm[];
                          struct sample_id sample_id;
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  comm   is a string containing the new name of the
                         process.

              PERF_RECORD_EXIT
                  This record indicates a process exit event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, ppid;
                          u32    tid, ptid;
                          u64    time;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_THROTTLE, PERF_RECORD_UNTHROTTLE
                  This record indicates a throttle/unthrottle event.

                      struct {
                          struct perf_event_header header;
                          u64    time;
                          u64    id;
                          u64    stream_id;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_FORK
                  This record indicates a fork event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, ppid;
                          u32    tid, ptid;
                          u64    time;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_READ
                  This record indicates a read event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, tid;
                          struct read_format values;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_SAMPLE
                  This record indicates a sample.

                      struct {
                          struct perf_event_header header;
                          u64    sample_id;   /* if PERF_SAMPLE_IDENTIFIER */
                          u64    ip;          /* if PERF_SAMPLE_IP */
                          u32    pid, tid;    /* if PERF_SAMPLE_TID */
                          u64    time;        /* if PERF_SAMPLE_TIME */
                          u64    addr;        /* if PERF_SAMPLE_ADDR */
                          u64    id;          /* if PERF_SAMPLE_ID */
                          u64    stream_id;   /* if PERF_SAMPLE_STREAM_ID */
                          u32    cpu, res;    /* if PERF_SAMPLE_CPU */
                          u64    period;      /* if PERF_SAMPLE_PERIOD */
                          struct read_format v;
                                              /* if PERF_SAMPLE_READ */
                          u64    nr;          /* if PERF_SAMPLE_CALLCHAIN */
                          u64    ips[nr];     /* if PERF_SAMPLE_CALLCHAIN */
                          u32    size;        /* if PERF_SAMPLE_RAW */
                          char   data[size];  /* if PERF_SAMPLE_RAW */
                          u64    bnr;         /* if PERF_SAMPLE_BRANCH_STACK */
                          struct perf_branch_entry lbr[bnr];
                                              /* if PERF_SAMPLE_BRANCH_STACK */
                          u64    abi;         /* if PERF_SAMPLE_REGS_USER */
                          u64    regs[weight(mask)];
                                              /* if PERF_SAMPLE_REGS_USER */
                          u64    size;        /* if PERF_SAMPLE_STACK_USER */
                          char   data[size];  /* if PERF_SAMPLE_STACK_USER */
                          u64    dyn_size;    /* if PERF_SAMPLE_STACK_USER &&
                                                 size != 0 */
                          u64    weight;      /* if PERF_SAMPLE_WEIGHT */
                          u64    data_src;    /* if PERF_SAMPLE_DATA_SRC */
                          u64    transaction; /* if PERF_SAMPLE_TRANSACTION */
                          u64    abi;         /* if PERF_SAMPLE_REGS_INTR */
                          u64    regs[weight(mask)];
                                              /* if PERF_SAMPLE_REGS_INTR */
                          u64    phys_addr;   /* if PERF_SAMPLE_PHYS_ADDR */
                          u64    cgroup;      /* if PERF_SAMPLE_CGROUP */
                      };

                  sample_id
                      If PERF_SAMPLE_IDENTIFIER is enabled, a 64-bit
                      unique ID is included.  This is a duplication of
                      the PERF_SAMPLE_ID id value, but included at the
                      beginning of the sample so parsers can easily
                      obtain the value.

                  ip  If PERF_SAMPLE_IP is enabled, then a 64-bit
                      instruction pointer value is included.

                  pid, tid
                      If PERF_SAMPLE_TID is enabled, then a 32-bit
                      process ID and 32-bit thread ID are included.

                  time
                      If PERF_SAMPLE_TIME is enabled, then a 64-bit
                      timestamp is included.  This is obtained via
                      local_clock() which is a hardware timestamp if
                      available and the jiffies value if not.

                  addr
                      If PERF_SAMPLE_ADDR is enabled, then a 64-bit
                      address is included.  This is usually the address
                      of a tracepoint, breakpoint, or software event;
                      otherwise the value is 0.

                  id  If PERF_SAMPLE_ID is enabled, a 64-bit unique ID
                      is included.  If the event is a member of an event
                      group, the group leader ID is returned.  This ID
                      is the same as the one returned by PERF_FORMAT_ID.

                  stream_id
                      If PERF_SAMPLE_STREAM_ID is enabled, a 64-bit
                      unique ID is included.  Unlike PERF_SAMPLE_ID the
                      actual ID is returned, not the group leader.  This
                      ID is the same as the one returned by
                      PERF_FORMAT_ID.

                  cpu, res
                      If PERF_SAMPLE_CPU is enabled, this is a 32-bit
                      value indicating which CPU was being used, in
                      addition to a reserved (unused) 32-bit value.

                  period
                      If PERF_SAMPLE_PERIOD is enabled, a 64-bit value
                      indicating the current sampling period is written.

                  v   If PERF_SAMPLE_READ is enabled, a structure of
                      type read_format is included which has values for
                      all events in the event group.  The values
                      included depend on the read_format value used at
                      perf_event_open() time.

                  nr, ips[nr]
                      If PERF_SAMPLE_CALLCHAIN is enabled, then a 64-bit
                      number is included which indicates how many
                      following 64-bit instruction pointers will follow.
                      This is the current callchain.

                  size, data[size]
                      If PERF_SAMPLE_RAW is enabled, then a 32-bit value
                      indicating size is included followed by an array
                      of 8-bit values of length size.  The values are
                      padded with 0 to have 64-bit alignment.

                      This RAW record data is opaque with respect to the
                      ABI.  The ABI doesn't make any promises with
                      respect to the stability of its content, it may
                      vary depending on event, hardware, and kernel
                      version.

                  bnr, lbr[bnr]
                      If PERF_SAMPLE_BRANCH_STACK is enabled, then a
                      64-bit value indicating the number of records is
                      included, followed by bnr perf_branch_entry
                      structures which each include the fields:

                      from   This indicates the source instruction (may
                             not be a branch).

                      to     The branch target.

                      mispred
                             The branch target was mispredicted.

                      predicted
                             The branch target was predicted.

                      in_tx (since Linux 3.11)
                             The branch was in a transactional memory
                             transaction.

                      abort (since Linux 3.11)
                             The branch was in an aborted transactional
                             memory transaction.

                      cycles (since Linux 4.3)
                             This reports the number of cycles elapsed
                             since the previous branch stack update.

                      The entries are from most to least recent, so the
                      first entry has the most recent branch.

                      Support for mispred, predicted, and cycles is
                      optional; if not supported, those values will be
                      0.

                      The type of branches recorded is specified by the
                      branch_sample_type field.

                  abi, regs[weight(mask)]
                      If PERF_SAMPLE_REGS_USER is enabled, then the user
                      CPU registers are recorded.

                      The abi field is one of PERF_SAMPLE_REGS_ABI_NONE,
                      PERF_SAMPLE_REGS_ABI_32, or
                      PERF_SAMPLE_REGS_ABI_64.

                      The regs field is an array of the CPU registers
                      that were specified by the sample_regs_user attr
                      field.  The number of values is the number of bits
                      set in the sample_regs_user bit mask.

                  size, data[size], dyn_size
                      If PERF_SAMPLE_STACK_USER is enabled, then the
                      user stack is recorded.  This can be used to
                      generate stack backtraces.  size is the size
                      requested by the user in sample_stack_user or else
                      the maximum record size.  data is the stack data
                      (a raw dump of the memory pointed to by the stack
                      pointer at the time of sampling).  dyn_size is the
                      amount of data actually dumped (can be less than
                      size).  Note that dyn_size is omitted if size is
                      0.

                  weight
                      If PERF_SAMPLE_WEIGHT is enabled, then a 64-bit
                      value provided by the hardware is recorded that
                      indicates how costly the event was.  This allows
                      expensive events to stand out more clearly in
                      profiles.

                  data_src
                      If PERF_SAMPLE_DATA_SRC is enabled, then a 64-bit
                      value is recorded that is made up of the following
                      fields:

                      mem_op
                          Type of opcode, a bitwise combination of:

                          PERF_MEM_OP_NA
                                 Not available
                          PERF_MEM_OP_LOAD
                                 Load instruction
                          PERF_MEM_OP_STORE
                                 Store instruction
                          PERF_MEM_OP_PFETCH
                                 Prefetch
                          PERF_MEM_OP_EXEC
                                 Executable code

                      mem_lvl
                          Memory hierarchy level hit or miss, a bitwise
                          combination of the following, shifted left by
                          PERF_MEM_LVL_SHIFT:

                          PERF_MEM_LVL_NA
                                 Not available
                          PERF_MEM_LVL_HIT
                                 Hit
                          PERF_MEM_LVL_MISS
                                 Miss
                          PERF_MEM_LVL_L1
                                 Level 1 cache
                          PERF_MEM_LVL_LFB
                                 Line fill buffer
                          PERF_MEM_LVL_L2
                                 Level 2 cache
                          PERF_MEM_LVL_L3
                                 Level 3 cache
                          PERF_MEM_LVL_LOC_RAM
                                 Local DRAM
                          PERF_MEM_LVL_REM_RAM1
                                 Remote DRAM 1 hop
                          PERF_MEM_LVL_REM_RAM2
                                 Remote DRAM 2 hops
                          PERF_MEM_LVL_REM_CCE1
                                 Remote cache 1 hop
                          PERF_MEM_LVL_REM_CCE2
                                 Remote cache 2 hops
                          PERF_MEM_LVL_IO
                                 I/O memory
                          PERF_MEM_LVL_UNC
                                 Uncached memory

                      mem_snoop
                          Snoop mode, a bitwise combination of the
                          following, shifted left by
                          PERF_MEM_SNOOP_SHIFT:

                          PERF_MEM_SNOOP_NA
                                 Not available
                          PERF_MEM_SNOOP_NONE
                                 No snoop
                          PERF_MEM_SNOOP_HIT
                                 Snoop hit
                          PERF_MEM_SNOOP_MISS
                                 Snoop miss
                          PERF_MEM_SNOOP_HITM
                                 Snoop hit modified

                      mem_lock
                          Lock instruction, a bitwise combination of the
                          following, shifted left by
                          PERF_MEM_LOCK_SHIFT:

                          PERF_MEM_LOCK_NA
                                 Not available
                          PERF_MEM_LOCK_LOCKED
                                 Locked transaction

                      mem_dtlb
                          TLB access hit or miss, a bitwise combination
                          of the following, shifted left by
                          PERF_MEM_TLB_SHIFT:

                          PERF_MEM_TLB_NA
                                 Not available
                          PERF_MEM_TLB_HIT
                                 Hit
                          PERF_MEM_TLB_MISS
                                 Miss
                          PERF_MEM_TLB_L1
                                 Level 1 TLB
                          PERF_MEM_TLB_L2
                                 Level 2 TLB
                          PERF_MEM_TLB_WK
                                 Hardware walker
                          PERF_MEM_TLB_OS
                                 OS fault handler

                  transaction
                      If the PERF_SAMPLE_TRANSACTION flag is set, then a
                      64-bit field is recorded describing the sources of
                      any transactional memory aborts.

                      The field is a bitwise combination of the
                      following values:

                      PERF_TXN_ELISION
                             Abort from an elision type transaction
                             (Intel-CPU-specific).

                      PERF_TXN_TRANSACTION
                             Abort from a generic transaction.

                      PERF_TXN_SYNC
                             Synchronous abort (related to the reported
                             instruction).

                      PERF_TXN_ASYNC
                             Asynchronous abort (not related to the
                             reported instruction).

                      PERF_TXN_RETRY
                             Retryable abort (retrying the transaction
                             may have succeeded).

                      PERF_TXN_CONFLICT
                             Abort due to memory conflicts with other
                             threads.

                      PERF_TXN_CAPACITY_WRITE
                             Abort due to write capacity overflow.

                      PERF_TXN_CAPACITY_READ
                             Abort due to read capacity overflow.

                      In addition, a user-specified abort code can be
                      obtained from the high 32 bits of the field by
                      shifting right by PERF_TXN_ABORT_SHIFT and masking
                      with the value PERF_TXN_ABORT_MASK.

                  abi, regs[weight(mask)]
                      If PERF_SAMPLE_REGS_INTR is enabled, then the user
                      CPU registers are recorded.

                      The abi field is one of PERF_SAMPLE_REGS_ABI_NONE,
                      PERF_SAMPLE_REGS_ABI_32, or
                      PERF_SAMPLE_REGS_ABI_64.

                      The regs field is an array of the CPU registers
                      that were specified by the sample_regs_intr attr
                      field.  The number of values is the number of bits
                      set in the sample_regs_intr bit mask.

                  phys_addr
                      If the PERF_SAMPLE_PHYS_ADDR flag is set, then the
                      64-bit physical address is recorded.

                  cgroup
                      If the PERF_SAMPLE_CGROUP flag is set, then the
                      64-bit cgroup ID (for the perf_event subsystem) is
                      recorded.  To get the pathname of the cgroup, the
                      ID should match to one in a PERF_RECORD_CGROUP .

              PERF_RECORD_MMAP2
                  This record includes extended information on mmap(2)
                  calls returning executable mappings.  The format is
                  similar to that of the PERF_RECORD_MMAP record, but
                  includes extra values that allow uniquely identifying
                  shared mappings.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                          u64    addr;
                          u64    len;
                          u64    pgoff;
                          u32    maj;
                          u32    min;
                          u64    ino;
                          u64    ino_generation;
                          u32    prot;
                          u32    flags;
                          char   filename[];
                          struct sample_id sample_id;
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  addr   is the address of the allocated memory.

                  len    is the length of the allocated memory.

                  pgoff  is the page offset of the allocated memory.

                  maj    is the major ID of the underlying device.

                  min    is the minor ID of the underlying device.

                  ino    is the inode number.

                  ino_generation
                         is the inode generation.

                  prot   is the protection information.

                  flags  is the flags information.

                  filename
                         is a string describing the backing of the
                         allocated memory.

              PERF_RECORD_AUX (since Linux 4.1)
                  This record reports that new data is available in the
                  separate AUX buffer region.

                      struct {
                          struct perf_event_header header;
                          u64    aux_offset;
                          u64    aux_size;
                          u64    flags;
                          struct sample_id sample_id;
                      };

                  aux_offset
                         offset in the AUX mmap region where the new
                         data begins.

                  aux_size
                         size of the data made available.

                  flags  describes the AUX update.

                         PERF_AUX_FLAG_TRUNCATED
                                if set, then the data returned was
                                truncated to fit the available buffer
                                size.

                         PERF_AUX_FLAG_OVERWRITE
                                if set, then the data returned has
                                overwritten previous data.

              PERF_RECORD_ITRACE_START (since Linux 4.1)
                  This record indicates which process has initiated an
                  instruction trace event, allowing tools to properly
                  correlate the instruction addresses in the AUX buffer
                  with the proper executable.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                      };

                  pid    process ID of the thread starting an
                         instruction trace.

                  tid    thread ID of the thread starting an instruction
                         trace.

              PERF_RECORD_LOST_SAMPLES (since Linux 4.2)
                  When using hardware sampling (such as Intel PEBS) this
                  record indicates some number of samples that may have
                  been lost.

                      struct {
                          struct perf_event_header header;
                          u64    lost;
                          struct sample_id sample_id;
                      };

                  lost   the number of potentially lost samples.

              PERF_RECORD_SWITCH (since Linux 4.3)
                  This record indicates a context switch has happened.
                  The PERF_RECORD_MISC_SWITCH_OUT bit in the misc field
                  indicates whether it was a context switch into or away
                  from the current process.

                      struct {
                          struct perf_event_header header;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_SWITCH_CPU_WIDE (since Linux 4.3)
                  As with PERF_RECORD_SWITCH this record indicates a
                  context switch has happened, but it only occurs when
                  sampling in CPU-wide mode and provides additional
                  information on the process being switched to/from.
                  The PERF_RECORD_MISC_SWITCH_OUT bit in the misc field
                  indicates whether it was a context switch into or away
                  from the current process.

                      struct {
                          struct perf_event_header header;
                          u32 next_prev_pid;
                          u32 next_prev_tid;
                          struct sample_id sample_id;
                      };

                  next_prev_pid
                         The process ID of the previous (if switching
                         in) or next (if switching out) process on the
                         CPU.

                  next_prev_tid
                         The thread ID of the previous (if switching in)
                         or next (if switching out) thread on the CPU.

              PERF_RECORD_NAMESPACES (since Linux 4.11)
                  This record includes various namespace information of
                  a process.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                          u64    nr_namespaces;
                          struct { u64 dev, inode } [nr_namespaces];
                          struct sample_id sample_id;
                      };

                  pid    is the process ID

                  tid    is the thread ID

                  nr_namespace
                         is the number of namespaces in this record

                  Each namespace has dev and inode fields and is
                  recorded in the fixed position like below:

                  NET_NS_INDEX=0
                         Network namespace

                  UTS_NS_INDEX=1
                         UTS namespace

                  IPC_NS_INDEX=2
                         IPC namespace

                  PID_NS_INDEX=3
                         PID namespace

                  USER_NS_INDEX=4
                         User namespace

                  MNT_NS_INDEX=5
                         Mount namespace

                  CGROUP_NS_INDEX=6
                         Cgroup namespace

              PERF_RECORD_KSYMBOL (since Linux 5.0)
                  This record indicates kernel symbol
                  register/unregister events.

                      struct {
                          struct perf_event_header header;
                          u64    addr;
                          u32    len;
                          u16    ksym_type;
                          u16    flags;
                          char   name[];
                          struct sample_id sample_id;
                      };

                  addr   is the address of the kernel symbol.

                  len    is the length of the kernel symbol.

                  ksym_type
                         is the type of the kernel symbol.  Currently
                         the following types are available:

                         PERF_RECORD_KSYMBOL_TYPE_BPF
                                The kernel symbol is a BPF function.

                  flags  If the PERF_RECORD_KSYMBOL_FLAGS_UNREGISTER is
                         set, then this event is for unregistering the
                         kernel symbol.

              PERF_RECORD_BPF_EVENT (since Linux 5.0)
                  This record indicates BPF program is loaded or
                  unloaded.

                      struct {
                          struct perf_event_header header;
                          u16 type;
                          u16 flags;
                          u32 id;
                          u8 tag[BPF_TAG_SIZE];
                          struct sample_id sample_id;
                      };

                  type   is one of the following values:

                         PERF_BPF_EVENT_PROG_LOAD
                                A BPF program is loaded

                         PERF_BPF_EVENT_PROG_UNLOAD
                                A BPF program is unloaded

                  id     is the ID of the BPF program.

                  tag    is the tag of the BPF program.  Currently,
                         BPF_TAG_SIZE is defined as 8.

              PERF_RECORD_CGROUP (since Linux 5.7)
                  This record indicates a new cgroup is created and
                  activated.

                      struct {
                          struct perf_event_header header;
                          u64    id;
                          char   path[];
                          struct sample_id sample_id;
                      };

                  id     is the cgroup identifier.  This can be also
                         retrieved by name_to_handle_at(2) on the cgroup
                         path (as a file handle).

                  path   is the path of the cgroup from the root.

              PERF_RECORD_TEXT_POKE (since Linux 5.8)
                  This record indicates a change in the kernel text.
                  This includes addition and removal of the text and the
                  corresponding length is zero in this case.

                      struct {
                          struct perf_event_header header;
                          u64    addr;
                          u16    old_len;
                          u16    new_len;
                          u8     bytes[];
                          struct sample_id sample_id;
                      };

                  addr   is the address of the change

                  old_len
                         is the old length

                  new_len
                         is the new length

                  bytes  contains old bytes immediately followed by new
                         bytes.

   Overflow handling
       Events can be set to notify when a threshold is crossed,
       indicating an overflow.  Overflow conditions can be captured by
       monitoring the event file descriptor with poll(2), select(2), or
       epoll(7).  Alternatively, the overflow events can be captured via
       sa signal handler, by enabling I/O signaling on the file
       descriptor; see the discussion of the F_SETOWN and F_SETSIG
       operations in fcntl(2).

       Overflows are generated only by sampling events (sample_period
       must have a nonzero value).

       There are two ways to generate overflow notifications.

       The first is to set a wakeup_events or wakeup_watermark value
       that will trigger if a certain number of samples or bytes have
       been written to the mmap ring buffer.  In this case, POLL_IN is
       indicated.

       The other way is by use of the PERF_EVENT_IOC_REFRESH ioctl.
       This ioctl adds to a counter that decrements each time the event
       overflows.  When nonzero, POLL_IN is indicated, but once the
       counter reaches 0 POLL_HUP is indicated and the underlying event
       is disabled.

       Refreshing an event group leader refreshes all siblings and
       refreshing with a parameter of 0 currently enables infinite
       refreshes; these behaviors are unsupported and should not be
       relied on.

       Starting with Linux 3.18, POLL_HUP is indicated if the event
       being monitored is attached to a different process and that
       process exits.

   rdpmc instruction
       Starting with Linux 3.4 on x86, you can use the rdpmc instruction
       to get low-latency reads without having to enter the kernel.
       Note that using rdpmc is not necessarily faster than other
       methods for reading event values.

       Support for this can be detected with the cap_usr_rdpmc field in
       the mmap page; documentation on how to calculate event values can
       be found in that section.

       Originally, when rdpmc support was enabled, any process (not just
       ones with an active perf event) could use the rdpmc instruction
       to access the counters.  Starting with Linux 4.0, rdpmc support
       is only allowed if an event is currently enabled in a process's
       context.  To restore the old behavior, write the value 2 to
       /sys/devices/cpu/rdpmc.

   perf_event ioctl calls
       Various ioctls act on perf_event_open() file descriptors:

       PERF_EVENT_IOC_ENABLE
              This enables the individual event or event group specified
              by the file descriptor argument.

              If the PERF_IOC_FLAG_GROUP bit is set in the ioctl
              argument, then all events in a group are enabled, even if
              the event specified is not the group leader (but see
              BUGS).

       PERF_EVENT_IOC_DISABLE
              This disables the individual counter or event group
              specified by the file descriptor argument.

              Enabling or disabling the leader of a group enables or
              disables the entire group; that is, while the group leader
              is disabled, none of the counters in the group will count.
              Enabling or disabling a member of a group other than the
              leader affects only that counter; disabling a non-leader
              stops that counter from counting but doesn't affect any
              other counter.

              If the PERF_IOC_FLAG_GROUP bit is set in the ioctl
              argument, then all events in a group are disabled, even if
              the event specified is not the group leader (but see
              BUGS).

       PERF_EVENT_IOC_REFRESH
              Non-inherited overflow counters can use this to enable a
              counter for a number of overflows specified by the
              argument, after which it is disabled.  Subsequent calls of
              this ioctl add the argument value to the current count.
              An overflow notification with POLL_IN set will happen on
              each overflow until the count reaches 0; when that happens
              a notification with POLL_HUP set is sent and the event is
              disabled.  Using an argument of 0 is considered undefined
              behavior.

       PERF_EVENT_IOC_RESET
              Reset the event count specified by the file descriptor
              argument to zero.  This resets only the counts; there is
              no way to reset the multiplexing time_enabled or
              time_running values.

              If the PERF_IOC_FLAG_GROUP bit is set in the ioctl
              argument, then all events in a group are reset, even if
              the event specified is not the group leader (but see
              BUGS).

       PERF_EVENT_IOC_PERIOD
              This updates the overflow period for the event.

              Since Linux 3.7 (on ARM) and Linux 3.14 (all other
              architectures), the new period takes effect immediately.
              On older kernels, the new period did not take effect until
              after the next overflow.

              The argument is a pointer to a 64-bit value containing the
              desired new period.

              Prior to Linux 2.6.36, this ioctl always failed due to a
              bug in the kernel.

       PERF_EVENT_IOC_SET_OUTPUT
              This tells the kernel to report event notifications to the
              specified file descriptor rather than the default one.
              The file descriptors must all be on the same CPU.

              The argument specifies the desired file descriptor, or -1
              if output should be ignored.

       PERF_EVENT_IOC_SET_FILTER (since Linux 2.6.33)
              This adds an ftrace filter to this event.

              The argument is a pointer to the desired ftrace filter.

       PERF_EVENT_IOC_ID (since Linux 3.12)
              This returns the event ID value for the given event file
              descriptor.

              The argument is a pointer to a 64-bit unsigned integer to
              hold the result.

       PERF_EVENT_IOC_SET_BPF (since Linux 4.1)
              This allows attaching a Berkeley Packet Filter (BPF)
              program to an existing kprobe tracepoint event.  You need
              CAP_PERFMON (since Linux 5.8) or CAP_SYS_ADMIN privileges
              to use this ioctl.

              The argument is a BPF program file descriptor that was
              created by a previous bpf(2) system call.

       PERF_EVENT_IOC_PAUSE_OUTPUT (since Linux 4.7)
              This allows pausing and resuming the event's ring-buffer.
              A paused ring-buffer does not prevent generation of
              samples, but simply discards them.  The discarded samples
              are considered lost, and cause a PERF_RECORD_LOST sample
              to be generated when possible.  An overflow signal may
              still be triggered by the discarded sample even though the
              ring-buffer remains empty.

              The argument is an unsigned 32-bit integer.  A nonzero
              value pauses the ring-buffer, while a zero value resumes
              the ring-buffer.

       PERF_EVENT_MODIFY_ATTRIBUTES (since Linux 4.17)
              This allows modifying an existing event without the
              overhead of closing and reopening a new event.  Currently
              this is supported only for breakpoint events.

              The argument is a pointer to a perf_event_attr structure
              containing the updated event settings.

       PERF_EVENT_IOC_QUERY_BPF (since Linux 4.16)
              This allows querying which Berkeley Packet Filter (BPF)
              programs are attached to an existing kprobe tracepoint.
              You can only attach one BPF program per event, but you can
              have multiple events attached to a tracepoint.  Querying
              this value on one tracepoint event returns the ID of all
              BPF programs in all events attached to the tracepoint.
              You need CAP_PERFMON (since Linux 5.8) or CAP_SYS_ADMIN
              privileges to use this ioctl.

              The argument is a pointer to a structure
                  struct perf_event_query_bpf {
                      __u32    ids_len;
                      __u32    prog_cnt;
                      __u32    ids[0];
                  };

              The ids_len field indicates the number of ids that can fit
              in the provided ids array.  The prog_cnt value is filled
              in by the kernel with the number of attached BPF programs.
              The ids array is filled with the ID of each attached BPF
              program.  If there are more programs than will fit in the
              array, then the kernel will return ENOSPC and ids_len will
              indicate the number of program IDs that were successfully
              copied.

   Using prctl(2)
       A process can enable or disable all currently open event groups
       using the prctl(2) PR_TASK_PERF_EVENTS_ENABLE and
       PR_TASK_PERF_EVENTS_DISABLE operations.  This applies only to
       events created locally by the calling process.  This does not
       apply to events created by other processes attached to the
       calling process or inherited events from a parent process.  Only
       group leaders are enabled and disabled, not any other members of
       the groups.

   perf_event related configuration files
       Files in /proc/sys/kernel/

           /proc/sys/kernel/perf_event_paranoid
                  The perf_event_paranoid file can be set to restrict
                  access to the performance counters.

                  2   allow only user-space measurements (default since
                      Linux 4.6).
                  1   allow both kernel and user measurements (default
                      before Linux 4.6).
                  0   allow access to CPU-specific data but not raw
                      tracepoint samples.
                  -1  no restrictions.

                  The existence of the perf_event_paranoid file is the
                  official method for determining if a kernel supports
                  perf_event_open().

           /proc/sys/kernel/perf_event_max_sample_rate
                  This sets the maximum sample rate.  Setting this too
                  high can allow users to sample at a rate that impacts
                  overall machine performance and potentially lock up
                  the machine.  The default value is 100000 (samples per
                  second).

           /proc/sys/kernel/perf_event_max_stack
                  This file sets the maximum depth of stack frame
                  entries reported when generating a call trace.

           /proc/sys/kernel/perf_event_mlock_kb
                  Maximum number of pages an unprivileged user can
                  mlock(2).  The default is 516 (kB).

       Files in /sys/bus/event_source/devices/

           Since Linux 2.6.34, the kernel supports having multiple PMUs
           available for monitoring.  Information on how to program
           these PMUs can be found under /sys/bus/event_source/devices/.
           Each subdirectory corresponds to a different PMU.

           /sys/bus/event_source/devices/*/type (since Linux 2.6.38)
                  This contains an integer that can be used in the type
                  field of perf_event_attr to indicate that you wish to
                  use this PMU.

           /sys/bus/event_source/devices/cpu/rdpmc (since Linux 3.4)
                  If this file is 1, then direct user-space access to
                  the performance counter registers is allowed via the
                  rdpmc instruction.  This can be disabled by echoing 0
                  to the file.

                  As of Linux 4.0 the behavior has changed, so that 1
                  now means only allow access to processes with active
                  perf events, with 2 indicating the old allow-anyone-
                  access behavior.

           /sys/bus/event_source/devices/*/format/ (since Linux 3.4)
                  This subdirectory contains information on the
                  architecture-specific subfields available for
                  programming the various config fields in the
                  perf_event_attr struct.

                  The content of each file is the name of the config
                  field, followed by a colon, followed by a series of
                  integer bit ranges separated by commas.  For example,
                  the file event may contain the value config1:1,6-10,44
                  which indicates that event is an attribute that
                  occupies bits 1,6–10, and 44 of
                  perf_event_attr::config1.

           /sys/bus/event_source/devices/*/events/ (since Linux 3.4)
                  This subdirectory contains files with predefined
                  events.  The contents are strings describing the event
                  settings expressed in terms of the fields found in the
                  previously mentioned ./format/ directory.  These are
                  not necessarily complete lists of all events supported
                  by a PMU, but usually a subset of events deemed useful
                  or interesting.

                  The content of each file is a list of attribute names
                  separated by commas.  Each entry has an optional value
                  (either hex or decimal).  If no value is specified,
                  then it is assumed to be a single-bit field with a
                  value of 1.  An example entry may look like this:
                  event=0x2,inv,ldlat=3.

           /sys/bus/event_source/devices/*/uevent
                  This file is the standard kernel device interface for
                  injecting hotplug events.

           /sys/bus/event_source/devices/*/cpumask (since Linux 3.7)
                  The cpumask file contains a comma-separated list of
                  integers that indicate a representative CPU number for
                  each socket (package) on the motherboard.  This is
                  needed when setting up uncore or northbridge events,
                  as those PMUs present socket-wide events.

RETURN VALUE         top

       On success, perf_event_open() returns the new file descriptor.
       On error, -1 is returned and errno is set to indicate the error.

ERRORS         top

       The errors returned by perf_event_open() can be inconsistent, and
       may vary across processor architectures and performance
       monitoring units.

       E2BIG  Returned if the perf_event_attr size value is too small
              (smaller than PERF_ATTR_SIZE_VER0), too big (larger than
              the page size), or larger than the kernel supports and the
              extra bytes are not zero.  When E2BIG is returned, the
              perf_event_attr size field is overwritten by the kernel to
              be the size of the structure it was expecting.

       EACCES Returned when the requested event requires CAP_PERFMON
              (since Linux 5.8) or CAP_SYS_ADMIN permissions (or a more
              permissive perf_event paranoid setting).  Some common
              cases where an unprivileged process may encounter this
              error: attaching to a process owned by a different user;
              monitoring all processes on a given CPU (i.e., specifying
              the pid argument as -1); and not setting exclude_kernel
              when the paranoid setting requires it.

       EBADF  Returned if the group_fd file descriptor is not valid, or,
              if PERF_FLAG_PID_CGROUP is set, the cgroup file descriptor
              in pid is not valid.

       EBUSY (since Linux 4.1)
              Returned if another event already has exclusive access to
              the PMU.

       EFAULT Returned if the attr pointer points at an invalid memory
              address.

       EINVAL Returned if the specified event is invalid.  There are
              many possible reasons for this.  A not-exhaustive list:
              sample_freq is higher than the maximum setting; the cpu to
              monitor does not exist; read_format is out of range;
              sample_type is out of range; the flags value is out of
              range; exclusive or pinned set and the event is not a
              group leader; the event config values are out of range or
              set reserved bits; the generic event selected is not
              supported; or there is not enough room to add the selected
              event.

       EINTR  Returned when trying to mix perf and ftrace handling for a
              uprobe.

       EMFILE Each opened event uses one file descriptor.  If a large
              number of events are opened, the per-process limit on the
              number of open file descriptors will be reached, and no
              more events can be created.

       ENODEV Returned when the event involves a feature not supported
              by the current CPU.

       ENOENT Returned if the type setting is not valid.  This error is
              also returned for some unsupported generic events.

       ENOSPC Prior to Linux 3.3, if there was not enough room for the
              event, ENOSPC was returned.  In Linux 3.3, this was
              changed to EINVAL.  ENOSPC is still returned if you try to
              add more breakpoint events than supported by the hardware.

       ENOSYS Returned if PERF_SAMPLE_STACK_USER is set in sample_type
              and it is not supported by hardware.

       EOPNOTSUPP
              Returned if an event requiring a specific hardware feature
              is requested but there is no hardware support.  This
              includes requesting low-skid events if not supported,
              branch tracing if it is not available, sampling if no PMU
              interrupt is available, and branch stacks for software
              events.

       EOVERFLOW (since Linux 4.8)
              Returned if PERF_SAMPLE_CALLCHAIN is requested and
              sample_max_stack is larger than the maximum specified in
              /proc/sys/kernel/perf_event_max_stack.

       EPERM  Returned on many (but not all) architectures when an
              unsupported exclude_hv, exclude_idle, exclude_user, or
              exclude_kernel setting is specified.

              It can also happen, as with EACCES, when the requested
              event requires CAP_PERFMON (since Linux 5.8) or
              CAP_SYS_ADMIN permissions (or a more permissive perf_event
              paranoid setting).  This includes setting a breakpoint on
              a kernel address, and (since Linux 3.13) setting a kernel
              function-trace tracepoint.

       ESRCH  Returned if attempting to attach to a process that does
              not exist.

VERSION         top

       perf_event_open() was introduced in Linux 2.6.31 but was called
       perf_counter_open().  It was renamed in Linux 2.6.32.

CONFORMING TO         top

       This perf_event_open() system call Linux-specific and should not
       be used in programs intended to be portable.

NOTES         top

       The official way of knowing if perf_event_open() support is
       enabled is checking for the existence of the file
       /proc/sys/kernel/perf_event_paranoid.

       CAP_PERFMON capability (since Linux 5.8) provides secure approach
       to performance monitoring and observability operations in a
       system according to the principal of least privilege (POSIX IEEE
       1003.1e).  Accessing system performance monitoring and
       observability operations using CAP_PERFMON rather than the much
       more powerful CAP_SYS_ADMIN excludes chances to misuse
       credentials and makes operations more secure.  CAP_SYS_ADMIN
       usage for secure system performance monitoring and observability
       is discouraged in favor of the CAP_PERFMON capability.

BUGS         top

       The F_SETOWN_EX option to fcntl(2) is needed to properly get
       overflow signals in threads.  This was introduced in Linux
       2.6.32.

       Prior to Linux 2.6.33 (at least for x86), the kernel did not
       check if events could be scheduled together until read time.  The
       same happens on all known kernels if the NMI watchdog is enabled.
       This means to see if a given set of events works you have to
       perf_event_open(), start, then read before you know for sure you
       can get valid measurements.

       Prior to Linux 2.6.34, event constraints were not enforced by the
       kernel.  In that case, some events would silently return "0" if
       the kernel scheduled them in an improper counter slot.

       Prior to Linux 2.6.34, there was a bug when multiplexing where
       the wrong results could be returned.

       Kernels from Linux 2.6.35 to Linux 2.6.39 can quickly crash the
       kernel if "inherit" is enabled and many threads are started.

       Prior to Linux 2.6.35, PERF_FORMAT_GROUP did not work with
       attached processes.

       There is a bug in the kernel code between Linux 2.6.36 and Linux
       3.0 that ignores the "watermark" field and acts as if a
       wakeup_event was chosen if the union has a nonzero value in it.

       From Linux 2.6.31 to Linux 3.4, the PERF_IOC_FLAG_GROUP ioctl
       argument was broken and would repeatedly operate on the event
       specified rather than iterating across all sibling events in a
       group.

       From Linux 3.4 to Linux 3.11, the mmap cap_usr_rdpmc and
       cap_usr_time bits mapped to the same location.  Code should
       migrate to the new cap_user_rdpmc and cap_user_time fields
       instead.

       Always double-check your results!  Various generalized events
       have had wrong values.  For example, retired branches measured
       the wrong thing on AMD machines until Linux 2.6.35.

EXAMPLES         top

       The following is a short example that measures the total
       instruction count of a call to printf(3).

       #include <stdlib.h>
       #include <stdio.h>
       #include <unistd.h>
       #include <string.h>
       #include <sys/ioctl.h>
       #include <linux/perf_event.h>
       #include <asm/unistd.h>

       static long
       perf_event_open(struct perf_event_attr *hw_event, pid_t pid,
                       int cpu, int group_fd, unsigned long flags)
       {
           int ret;

           ret = syscall(__NR_perf_event_open, hw_event, pid, cpu,
                          group_fd, flags);
           return ret;
       }

       int
       main(int argc, char **argv)
       {
           struct perf_event_attr pe;
           long long count;
           int fd;

           memset(&pe, 0, sizeof(pe));
           pe.type = PERF_TYPE_HARDWARE;
           pe.size = sizeof(pe);
           pe.config = PERF_COUNT_HW_INSTRUCTIONS;
           pe.disabled = 1;
           pe.exclude_kernel = 1;
           pe.exclude_hv = 1;

           fd = perf_event_open(&pe, 0, -1, -1, 0);
           if (fd == -1) {
              fprintf(stderr, "Error opening leader %llx\n", pe.config);
              exit(EXIT_FAILURE);
           }

           ioctl(fd, PERF_EVENT_IOC_RESET, 0);
           ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);

           printf("Measuring instruction count for this printf\n");

           ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
           read(fd, &count, sizeof(count));

           printf("Used %lld instructions\n", count);

           close(fd);
       }

SEE ALSO         top

       perf(1), fcntl(2), mmap(2), open(2), prctl(2), read(2)

       Documentation/admin-guide/perf-security.rst in the kernel source
       tree

COLOPHON         top

       This page is part of release 5.12 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22             PERF_EVENT_OPEN(2)

Pages that refer to this page: bpf(2)gettid(2)openat2(2)syscalls(2)stapprobes(3stap)proc(5)systemd.exec(5)bpf-helpers(7)capabilities(7)cgroups(7)