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_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).

              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 */
                     __reserved_1   : 39;

               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 */
               __u64 sample_regs_intr;     /* regs to dump on samples */

       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:

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

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

                     This indicates a tracepoint provided by the kernel
                     tracepoint infrastructure.

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

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

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

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

       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

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

       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

              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

                          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
                          conjunction 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

                          Mispredicted branch instructions.

                          Bus cycles, which can be different from total

                   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

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

                          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,
                          after 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
                          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:

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

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

              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.

                     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
                     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

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

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

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

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

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

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

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

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

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

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

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

              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
                     overcommitted and multiplexing is happening.

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

              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.

              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).

              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

              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 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 as
              well as writing to /proc/self/comm.  If the comm_exec flag is
              also successfully set (possible since Linux 3.16), then the
              misc flag PERF_RECORD_MISC_COMM_EXEC can be used to
              differentiate the 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
              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

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

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

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

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

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

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

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

              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
              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,
              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.

                     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.

                     Any return branch.

                     Indirect calls.

              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.

       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

       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
              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
       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 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

       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;
                      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
              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.

              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

                  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

                  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);

              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().

              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 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

       size   This indicates the size of the record.

       misc   The misc field contains additional information about the

              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):

                     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.

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

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

              PERF_RECORD_MISC_COMM_EXEC (since Linux 3.16)
                     This is set for a PERF_RECORD_COMM record on kernels
                     more recent than Linux 3.16 if a process name change
                     was caused by an exec(2) system call.  It is an alias
                     for PERF_RECORD_MISC_MMAP_DATA since the two values
                     would not be set in the same record.

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

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

       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.

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

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

                  pid    is the process ID.

                  tid    is the thread ID.

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

                  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 */
                          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
                      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.

                      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.

                      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.

                      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
                      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()

                  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.

                             The branch target was mispredicted.

                             The branch target was predicted.

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

                      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 the user
                      stack is recorded.  This can be used to generate stack
                      backtraces.  size is the size requested by the user in
                      sample_stack_user or else the maximum record size.
                      data is the stack data (a raw dump of the memory
                      pointed to by the stack pointer at the time of
                      sampling).  dyn_size is the amount of data actually
                      dumped (can be less than size).

                      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
                          combination 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
                          following, shifted left by PERF_MEM_SNOOP_SHIFT:

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

                          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

                             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
                      obtained from the high 32 bits of the field by
                      shifting right by PERF_TXN_ABORT_SHIFT and masking
                      with 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

   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(2).
       Alternately, a SIGIO signal handler can be created and the event
       configured with fcntl(2) to generate SIGIO signals.

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

       There are two ways to generate overflow notifications.

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

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

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

       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.

   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
              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

              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
              notification with POLL_IN set will happen on each overflow
              until the count reaches 0; when that happens a notification
              with POLL_HUP set is sent and the event is disabled.  Using an
              argument of 0 is considered undefined behavior.

              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

              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).

              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.

              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

              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/


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

                  2   allow only 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


                  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).


                  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

           /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:

                  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

       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.

              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

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

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.

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;

       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);

           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);


SEE ALSO         top

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

COLOPHON         top

       This page is part of release 4.02 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                            2015-07-23               PERF_EVENT_OPEN(2)