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

NAME         top

       perf_event_open - set up performance monitoring

SYNOPSIS         top

       #include <linux/perf_event.h>
       #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).

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

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

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

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

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

       pid == -1 and cpu >= 0
              This measures all processes/threads on the specified CPU.
              This requires CAP_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.

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

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

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

       PERF_FLAG_FD_NO_GROUP
              This flag allows creating an event as part of an event group
              but having no group leader.  It is unclear why this is useful.

       PERF_FLAG_FD_OUTPUT
              This flag reroutes the output from an event to the group
              leader.

       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 */
                     __reserved_1   : 41;

               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 config1;          /* extension of config */
               };

               union {
                   __u64 bp_len;           /* breakpoint length */
                   __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 */
               __u32 __reserved_2;         /* Align to u64 */

           };

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

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

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

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

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

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

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

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

              dynamic PMU
                     Since Linux 2.6.39, perf_event_open() can support
                     multiple PMUs.  To enable this, a value exported by the
                     kernel can be used in the type field to indicate which
                     PMU to use.  The value to use can be found in the sysfs
                     filesystem: there is a subdirectory per PMU instance
                     under /sys/bus/event_source/devices.  In each
                     subdirectory there is a type file whose content is an
                     integer that can be used in the type field.  For
                     instance, /sys/bus/event_source/devices/cpu/type
                     contains the value for the core CPU PMU, which is
                     usually 4.

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

              The related define PERF_ATTR_SIZE_VER0 is set to 64; this was
              the size of the first published struct.  PERF_ATTR_SIZE_VER1
              is 72, corresponding to the addition of breakpoints in Linux
              2.6.33.  PERF_ATTR_SIZE_VER2 is 80 corresponding to the
              addition of branch sampling in Linux 3.4.  PERF_ATR_SIZE_VER3
              is 96 corresponding to the addition of sample_regs_user and
              sample_stack_user in Linux 3.7.

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

              The most significant bit (bit 63) of config signifies CPU-
              specific (raw) counter configuration data; if the most
              significant bit is unset, the next 7 bits are an event type
              and the rest of the bits are the event identifier.

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

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

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

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

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

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

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

                   PERF_COUNT_HW_BRANCH_MISSES
                          Mispredicted branch instructions.

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

                   PERF_COUNT_HW_STALLED_CYCLES_FRONTEND (since Linux 3.0)
                          Stalled cycles during issue.

                   PERF_COUNT_HW_STALLED_CYCLES_BACKEND (since Linux 3.0)
                          Stalled cycles during retirement.

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

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

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

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

                   PERF_COUNT_SW_PAGE_FAULTS
                          This reports the number of page faults.

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

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

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

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

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

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

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

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

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

                      (perf_hw_cache_id) | (perf_hw_cache_op_id << 8) |
                      (perf_hw_cache_op_result_id << 16)

                  where perf_hw_cache_id is one of:

                      PERF_COUNT_HW_CACHE_L1D
                             for measuring Level 1 Data Cache

                      PERF_COUNT_HW_CACHE_L1I
                             for measuring Level 1 Instruction Cache

                      PERF_COUNT_HW_CACHE_LL
                             for measuring Last-Level Cache

                      PERF_COUNT_HW_CACHE_DTLB
                             for measuring the Data TLB

                      PERF_COUNT_HW_CACHE_ITLB
                             for measuring the Instruction TLB

                      PERF_COUNT_HW_CACHE_BPU
                             for measuring the branch prediction unit

                      PERF_COUNT_HW_CACHE_NODE (since Linux 3.0)
                             for measuring local memory accesses

                  and perf_hw_cache_op_id is one of

                      PERF_COUNT_HW_CACHE_OP_READ
                             for read accesses

                      PERF_COUNT_HW_CACHE_OP_WRITE
                             for write accesses

                      PERF_COUNT_HW_CACHE_OP_PREFETCH
                             for prefetch accesses

                  and perf_hw_cache_op_result_id is one of

                      PERF_COUNT_HW_CACHE_RESULT_ACCESS
                             to measure accesses

                      PERF_COUNT_HW_CACHE_RESULT_MISS
                             to measure misses

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

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

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

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

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

              PERF_SAMPLE_IP
                     Records instruction pointer.

              PERF_SAMPLE_TID
                     Records the process and thread IDs.

              PERF_SAMPLE_TIME
                     Records a timestamp.

              PERF_SAMPLE_ADDR
                     Records an address, if applicable.

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

              PERF_SAMPLE_CALLCHAIN
                     Records the callchain (stack backtrace).

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

              PERF_SAMPLE_CPU
                     Records CPU number.

              PERF_SAMPLE_PERIOD
                     Records the current sampling period.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       exclude_idle
              If set, don't count when the CPU is idle.

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

       comm   The comm bit enables tracking of process command name as
              modified by the exec(2) and prctl(PR_SET_NAME) system calls.
              Unfortunately for tools, there is no way to distinguish one
              system call versus the other.

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

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

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

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

       watermark
              If set, have a sampling interrupt happen when we cross the
              wakeup_watermark boundary.  Otherwise, interrupts happen after
              wakeup_events samples.

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

              The values of this 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
                     PERF_RECORD_MISC_EXACT_IP.

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

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

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

              The layout is described by this pseudo-structure:
                  struct sample_id {
                      { u32 pid, tid; } /* if PERF_SAMPLE_TID set        */
                      { u64 time;     } /* if PERF_SAMPLE_TIME set       */
                      { u64 id;       } /* if PERF_SAMPLE_ID set         */
                      { u64 stream_id;} /* if PERF_SAMPLE_STREAM_ID set  */
                      { u32 cpu, res; } /* if PERF_SAMPLE_CPU set        */
                      { u64 id;       } /* if PERF_SAMPLE_IDENTIFIER set */
                  };

       exclude_host (since Linux 3.2)
              Do not measure time spent in VM host.

       exclude_guest (since Linux 3.2)
              Do not measure time spent in VM guest.

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

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

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

              wakeup_events only counts PERF_RECORD_SAMPLE record types.  To
              receive a signal for every incoming PERF_RECORD type set
              wakeup_watermark to 1.

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

              HW_BREAKPOINT_EMPTY
                     No breakpoint.

              HW_BREAKPOINT_R
                     Count when we read the memory location.

              HW_BREAKPOINT_W
                     Count when we write the memory location.

              HW_BREAKPOINT_RW
                     Count when we read or write the memory location.

              HW_BREAKPOINT_X
                     Count when we execute code at the memory location.

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

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

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

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

       config2 (since Linux 2.6.39)

              config2 is a further extension of the config1 field.

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

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

              PERF_SAMPLE_BRANCH_USER
                     Branch target is in user space.

              PERF_SAMPLE_BRANCH_KERNEL
                     Branch target is in kernel space.

              PERF_SAMPLE_BRANCH_HV
                     Branch target is in hypervisor.

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

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

              PERF_SAMPLE_BRANCH_ANY
                     Any branch type.

              PERF_SAMPLE_BRANCH_ANY_CALL
                     Any call branch.

              PERF_SAMPLE_BRANCH_ANY_RETURN
                     Any return branch.

              PERF_SAMPLE_BRANCH_IND_CALL
                     Indirect calls.

              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.

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

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

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

       If you attempt to read into a buffer that is not big enough to hold
       the data ENOSPC is returned

       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.  Only available
              if PERF_FORMAT_GROUP was specified.

       time_enabled, time_running
              Total time the event was enabled and running.  Normally these
              are the same.  If more events are started, then available
              counter slots on the PMU, then multiplexing happens and events
              run only part of the time.  In that case, 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 there
              if PERF_FORMAT_ID was specified in read_format.

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

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

       Before kernel 2.6.39, there is a bug that means you must allocate a
       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 1k */
               __u64 data_head;         /* head in the data section */
               __u64 data_tail;         /* user-space written tail */
           }

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

       version
              Version number of this structure.

       compat_version
              The lowest version this is compatible with.

       lock   A seqlock for synchronization.

       index  A unique hardware counter identifier.

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

       time_enabled
              Time the event was active.

       time_running
              Time the event was running.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       time_shift, time_mult, time_offset

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

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

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

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

       time_zero (since Linux 3.12)

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

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

              And vice versa:

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

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

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

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

       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.  That way a newer
       perf.data file will be supported by older perf tools, with these new
       optional fields being ignored.

       The mmap values start with a header:

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

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

       size   This indicates the size of the record.

       misc   The misc field contains additional information about the
              sample.

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

              PERF_RECORD_MISC_CPUMODE_UNKNOWN
                     Unknown CPU mode.

              PERF_RECORD_MISC_KERNEL
                     Sample happened in the kernel.

              PERF_RECORD_MISC_USER
                     Sample happened in user code.

              PERF_RECORD_MISC_HYPERVISOR
                     Sample happened in the hypervisor.

              PERF_RECORD_MISC_GUEST_KERNEL
                     Sample happened in the guest kernel.

              PERF_RECORD_MISC_GUEST_USER
                     Sample happened in guest user code.

              In addition, one of the following bits can be set:

              PERF_RECORD_MISC_MMAP_DATA
                     This is set when the mapping is not executable;
                     otherwise the mapping is executable.

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

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

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

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

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

              PERF_RECORD_LOST
                  This record indicates when events are lost.

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

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

                  lost   is the number of events that were lost.

              PERF_RECORD_COMM
                  This record indicates a change in the process name.

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

              PERF_RECORD_EXIT
                  This record indicates a process exit event.

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

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

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

              PERF_RECORD_FORK
                  This record indicates a fork event.

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

              PERF_RECORD_READ
                  This record indicates a read event.

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

              PERF_RECORD_SAMPLE
                  This record indicates a sample.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

                      to     The branch target.

                      mispred
                             The branch target was mispredicted.

                      predicted
                             The branch target was predicted.

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

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

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

                      Support for mispred and predicted is optional; if not
                      supported, both 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 record the
                      user stack to enable backtracing.  size is the size
                      requested by the user in stack_user_size or else the
                      maximum record size.  data is the stack data.
                      dyn_size is the amount of data actually dumped (can be
                      less than size).

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

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

                      mem_op
                          Type of opcode, a bitwise combination of:

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

                      mem_lvl
                          Memory hierarchy level hit or miss, a bitwise
                          combination of:

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

                      mem_snoop
                          Snoop mode, a bitwise combination of:

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

                      mem_lock
                          Lock instruction, a bitwise combination of:

                          PERF_MEM_LOCK_NA        Not available
                          PERF_MEM_LOCK_LOCKED    Locked transaction

                      mem_dtlb
                          TLB access hit or miss, a bitwise combination of:

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

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

                      The field is a bitwise combination of the following
                      values:

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

                      PERF_TXN_TRANSACTION
                             Abort from a generic transaction.

                      PERF_TXN_SYNC
                             Synchronous abort (related to the reported
                             instruction).

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

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

                      PERF_TXN_CONFLICT
                             Abort due to memory conflicts with other
                             threads.

                      PERF_TXN_CAPACITY_WRITE
                             Abort due to write capacity overflow.

                      PERF_TXN_CAPACITY_READ
                             Abort due to read capacity overflow.

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

   Signal overflow
       Events can be set to deliver a signal when a threshold is crossed.
       The signal handler is set up using the poll(2), select(2), epoll(2)
       and fcntl(2), system calls.

       To generate signals, sampling must be enabled (sample_period must
       have a nonzero value).

       There are two ways to generate signals.

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

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

       Note: on newer kernels (definitely noticed with 3.2) a signal is
       provided for every overflow, even if wakeup_events is not set.

   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.

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

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

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

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

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

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

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

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

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

       PERF_EVENT_IOC_PERIOD
              This updates the overflow period for the event.

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

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

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

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

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

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

              The argument is a pointer to the desired ftrace filter.

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

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

   Using prctl
       A process can enable or disable all the event groups that are
       attached to it using the prctl(2) PR_TASK_PERF_EVENTS_ENABLE and
       PR_TASK_PERF_EVENTS_DISABLE operations.  This applies to all counters
       on the calling process, whether created by this process or by
       another, and does not affect any counters that this process has
       created on other processes.  It enables or disables only the group
       leaders, not any other members in the groups.

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

           /proc/sys/kernel/perf_event_paranoid

                  The perf_event_paranoid file can be set to restrict access
                  to the performance counters.

                  2   only allow user-space measurements.

                  1   allow both kernel and user measurements (default).

                  0   allow access to CPU-specific data but not raw
                      tracepoint samples.

                  -1  no restrictions.

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

           /proc/sys/kernel/perf_event_max_sample_rate

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

           /proc/sys/kernel/perf_event_mlock_kb

                  Maximum number of pages an unprivileged user can mlock(2).
                  The default is 516 (kB).

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

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

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

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

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

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

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

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

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

RETURN VALUE         top

       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_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.

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

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

       EMFILE Each opened event uses one file descriptor.  If a large number
              of events are opened the per-user file descriptor limit (often
              1024) will be hit and no more events can be created.

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

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

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

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

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

       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_SYS_ADMIN permissions (or a more permissive
              perf_event paranoid setting).  This includes setting a
              breakpoint on a kernel address, and (since Linux 3.13) setting
              a kernel function-trace tracepoint.

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

VERSION         top

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

CONFORMING TO         top

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

NOTES         top

       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
       /proc/sys/kernel/perf_event_paranoid.

BUGS         top

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

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

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

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

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

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

       In older Linux 2.6 versions, refreshing an event group leader
       refreshed all siblings, and refreshing with a parameter of 0 enabled
       infinite refresh.  This behavior is unsupported and should not be
       relied on.

       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.

EXAMPLE         top

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

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

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

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

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

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

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

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

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

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

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

           close(fd);
       }

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 3.70 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
       http://www.kernel.org/doc/man-pages/.

Linux                            2014-04-17               PERF_EVENT_OPEN(2)