perf_event_open(2) — Linux manual page


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>
       #include <linux/hw_breakpoint.h>

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

       Note: There is no glibc wrapper for this system call; see NOTES.

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).

       The pid and cpu arguments allow specifying which process and CPU to

       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

       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).

              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

       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 */

                     __reserved_1   : 37;

               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 de‐
       tail below:

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

                     This indicates one of the "generalized" hardware events
                     provided by the kernel.  See the config field defini‐
                     tion for more details.

                     This indicates one of the software-defined events pro‐
                     vided by the kernel (even if no hardware support is

                     This indicates a tracepoint provided by the kernel tra‐
                     cepoint infrastructure.

                     This indicates a hardware cache event.  This has a spe‐
                     cial encoding, described in the config field defini‐

                     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 ad‐
                     dress as well as execution of an instruction address.

              dynamic PMU
                     Since Linux 2.6.38, perf_event_open() can support mul‐
                     tiple 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 subdirec‐
                     tory 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 at‐
                     tach 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 com‐

              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 addi‐
              tion of branch sampling in Linux 3.4.  PERF_ATTR_SIZE_VER3 is
              96 corresponding to the addition of sample_regs_user and sam‐
              ple_stack_user in Linux 3.7.  PERF_ATTR_SIZE_VER4 is 104 cor‐
              responding 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 de‐

              There are various ways to set the config field that are depen‐
              dent on the value of the previously described type field.
              What follows are various possible settings for config sepa‐
              rated out by type.

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

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

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

                          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

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

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

                          Mispredicted branch instructions.

                          Bus cycles, which can be different from total cy‐

                   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 scal‐

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

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

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

                          This reports the number of page faults.

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

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

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

                          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 perfor‐
                          mance.  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 instruc‐
                          tions 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 hard‐
              ware CPU cache event.  To calculate the appropriate config
              value use the following equation:

                      (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:

                             for measuring Level 1 Data Cache

                             for measuring Level 1 Instruction Cache

                             for measuring Last-Level Cache

                             for measuring the Data TLB

                             for measuring the Instruction TLB

                             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:

                             for read accesses

                             for write accesses

                             for prefetch accesses

                  and perf_hw_cache_op_result_id is one of:

                             to measure accesses

                             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 li‐
              brary can be used to translate from the name in the architec‐
              tural 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_off‐
              set, 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 notifica‐
              tion every N events, where N is given by sample_period.  A
              sampling event has sample_period > 0.  When an overflow oc‐
              curs, requested data is recorded in the mmap buffer.  The sam‐
              ple_type field controls what data is recorded on each over‐

              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.

              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_sam‐
              ple_format order.

                     Records instruction pointer.

                     Records the process and thread IDs.

                     Records a timestamp.

                     Records an address, if applicable.

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

                     Records the callchain (stack backtrace).

                     Records a unique ID for the opened event's group

                     Records CPU number.

                     Records the current sampling period.

                     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

                     Records additional data, if applicable.  Usually re‐
                     turned 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 fea‐

                     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 hard‐
                     ware 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 dif‐
                     ferent 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 sam‐
                     ple 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 SAM‐
                     PLE_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 sup‐

                     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 count‐
                     ing, may cause extraneous aborts (by causing an inter‐
                     rupt 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_SAM‐
                     PLE_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 pre‐
                     cise_ip is set higher than zero then the register val‐
                     ues returned are those captured by hardware at the time
                     of the sampled instruction's retirement.

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

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

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

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

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

              The disabled bit specifies whether the counter starts out dis‐
              abled 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 en‐

              The inherit bit specifies that this counter should count
              events of child tasks as well as the task specified.  This ap‐
              plies 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 coun‐
              ters 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.

              The exclusive bit specifies that when this counter's group is
              on the CPU, it should be the only group using the CPU's coun‐
              ters.  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 en‐
              abled NMI Watchdog Timer interface).

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

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

              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.

              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 pro‐
              gram (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 modi‐
              fied by the exec(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 differen‐
              tiate the exec(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.

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

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

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

              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 instruc‐
              tions 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 lim‐
              ited 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

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

       sample_id_all (since Linux 2.6.38)
              If set, then TID, TIME, ID, STREAM_ID, and CPU can addition‐
              ally be included in non-PERF_RECORD_SAMPLEs if the correspond‐
              ing 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 mea‐
              sure 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 function‐
              ality 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 function‐
              ality 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 map‐
              pings.  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 exec(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 gener‐
              ated 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 trace‐
              point 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.

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

              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:

                     No breakpoint.

                     Count when we read the memory location.

                     Count when we write the memory location.

                     Count when we read or write the memory location.

                     Count when we execute code at the memory location.

              The values can be combined via a bitwise or, but the combina‐
              tion of HW_BREAKPOINT_R or HW_BREAKPOINT_W with HW_BREAK‐
              POINT_X is not allowed.

       bp_addr (since Linux 2.6.33)
              This is the address of the breakpoint.  For execution break‐
              points, this is the memory address of the instruction of in‐
              terest; for read and write breakpoints, it is the memory ad‐
              dress 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 OFF‐
              CORE_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,
              POINT_LEN_8.  For an execution breakpoint, set this to

       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 lev‐
              els do not have to match.

                     Branch target is in user space.

                     Branch target is in kernel space.

                     Branch target is in hypervisor.

                     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.

                     Any branch type.

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

                     Indirect calls.

              PERF_SAMPLE_BRANCH_CALL (since Linux 4.4)
                     Direct calls.

                     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 re‐
                     quires 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-spe‐
              cific and is described in the kernel header file arch/ARCH/in‐

       sample_stack_user (since Linux 3.7)
              This defines the size of the user stack to dump if PERF_SAM‐
              PLE_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_MONO‐
              rently 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

   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 struc‐
       ture 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

       The mmap size should be 1+2^n pages, where the first page is a meta‐
       data 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 number of this structure.

              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

              Time the event was active.

              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 in‐

       cap_bit0_is_deprecated (since Linux 3.12)
              If set, this bit indicates that the kernel supports the prop‐
              erly 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 coun‐
              ters 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;
                      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);

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

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

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

              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 simi‐

                  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

                  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);

              This points to the head of the data section.  The value con‐
              tinuously increases, it does not wrap.  The value needs to be
              manually wrapped by the size of the mmap buffer before access‐
              ing the samples.

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

              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

       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 sam‐
              ple buffer).  An example of a high-bandwidth stream is in‐
              struction tracing support, as is found in newer Intel proces‐

              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 off‐
              set 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 al‐

              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 mea‐
              surement 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 file to be supported by older perf tools, with the new op‐
       tional 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

       size   This indicates the size of the record.

       misc   The misc field contains additional information about the sam‐

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

                     Unknown CPU mode.

                     Sample happened in the kernel.

                     Sample happened in user code.

                     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; other‐
                     wise 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 exec(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:

                     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 (cur‐
                     rently not used).

                     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 corre‐
              sponding record (that follows the header) depend on the type
              selected as shown.

                  The MMAP events record the PROT_EXEC mappings so that we
                  can correlate user-space IPs to code.  They have the fol‐
                  lowing 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 mem‐

                  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   is the number of events that were lost.

                  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.

                  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;

                  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;

                  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;

                  This record indicates a read event.

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

                  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 */

                      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 instruc‐
                      tion 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.

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

                      If PERF_SAMPLE_ADDR is enabled, then a 64-bit address
                      is included.  This is usually the address of a trace‐
                      point, 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.

                      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.

                      If PERF_SAMPLE_PERIOD is enabled, a 64-bit value indi‐
                      cating 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()

                  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 in‐
                      dicating 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.

                             The branch target was mispredicted.

                             The branch target was predicted.

                      in_tx (since Linux 3.11)
                             The branch was in a transactional memory trans‐

                      abort (since Linux 3.11)
                             The branch was in an aborted transactional mem‐
                             ory 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 op‐
                      tional; 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 sam‐
                      pling).  dyn_size is the amount of data actually
                      dumped (can be less than size).  Note that dyn_size is
                      omitted if size is 0.

                      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.

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

                          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

                          Memory hierarchy level hit or miss, a bitwise com‐
                          bination of the following, shifted left by

                          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

                          Snoop mode, a bitwise combination of the follow‐
                          ing, 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

                          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

                          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

                      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

                             Abort from an elision type transaction (Intel-

                             Abort from a generic transaction.

                             Synchronous abort (related to the reported in‐

                             Asynchronous abort (not related to the reported

                             Retryable abort (retrying the transaction may
                             have succeeded).

                             Abort due to memory conflicts with other

                             Abort due to write capacity overflow.

                             Abort due to read capacity overflow.

                      In addition, a user-specified abort code can be ob‐
                      tained 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.

                  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.

                         is the inode generation.

                  prot   is the protection information.

                  flags  is the flags information.

                         is a string describing the backing of the allocated

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

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

                         offset in the AUX mmap region where the new data

                         size of the data made available.

                  flags  describes the AUX update.

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

                                if set, then the data returned has overwrit‐
                                ten previous data.

              PERF_RECORD_ITRACE_START (since Linux 4.1)
                  This record indicates which process has initiated an in‐
                  struction trace event, allowing tools to properly corre‐
                  late 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

                  tid    thread ID of the thread starting an instruction

              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

                      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 indi‐
                  cates 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 indi‐
                  cates 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;

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

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

   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).  Alterna‐
       tively, 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 writ‐
       ten 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 over‐
       flows.  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 refresh‐
       ing 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/de‐

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

              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).

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

              Enabling or disabling the leader of a group enables or dis‐
              ables the entire group; that is, while the group leader is
              disabled, none of the counters in the group will count.  En‐
              abling 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).

              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 no‐
              tification 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 ar‐
              gument of 0 is considered undefined behavior.

              Reset the event count specified by the file descriptor argu‐
              ment 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 speci‐
              fied is not the group leader (but see BUGS).

              This updates the overflow period for the event.

              Since Linux 3.7 (on ARM) and Linux 3.14 (all other architec‐
              tures), 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 de‐
              sired new period.

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

              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 de‐

              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

              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-

       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 sup‐
              ported only for breakpoint events.

              The argument is a pointer to a perf_event_attr structure con‐
              taining the updated event settings.

       PERF_EVENT_IOC_QUERY_BPF (since Linux 4.16)
              This allows querying which Berkeley Packet Filter (BPF) pro‐
              grams are attached to an existing kprobe tracepoint.  You can
              only attach one BPF program per event, but you can have multi‐
              ple 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

              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
       ABLE operations.  This applies only to events created locally by the
       calling process.  This does not apply to events created by other pro‐
       cesses 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/

                  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 be‐
                      fore Linux 4.6).
                  0   allow access to CPU-specific data but not raw trace‐
                      point samples.
                  -1  no restrictions.

                  The existence of the perf_event_paranoid file is the offi‐
                  cial method for determining if a kernel supports

                  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).

                  This file sets the maximum depth of stack frame entries
                  reported when generating a call trace.

                  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 in‐
                  struction.  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 be‐

           /sys/bus/event_source/devices/*/format/ (since Linux 3.4)
                  This subdirectory contains information on the architec‐
                  ture-specific subfields available for programming the var‐
                  ious 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 ex‐
                  pressed 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 usu‐
                  ally a subset of events deemed useful or interesting.

                  The content of each file is a list of attribute names sep‐
                  arated by commas.  Each entry has an optional value (ei‐
                  ther 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.

                  This file is the standard kernel device interface for in‐
                  jecting hotplug events.

           /sys/bus/event_source/devices/*/cpumask (since Linux 3.7)
                  The cpumask file contains a comma-separated list of inte‐
                  gers 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

       perf_event_open() returns the new file descriptor, or -1 if an error
       occurred (in which case, errno is set appropriately).

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

       EFAULT Returned if the attr pointer points at an invalid memory

       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

       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.

              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

       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

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

       Glibc does not provide a wrapper for this system call; call it using
       syscall(2).  See the example below.

       The official way of knowing if perf_event_open() support is enabled
       is checking for the existence of the file

       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

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

       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

       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;

       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);

           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);


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 5.09 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

Linux                            2020-11-01               PERF_EVENT_OPEN(2)

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