Ubuntu Manpage: perf_event_open - set up performance monitoring

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Provided by: manpages-dev_3.54-1ubuntu1_all

NAME

       perf_event_open - set up performance monitoring

SYNOPSIS

       #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

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 argument pid allows events to be attached to processes in various ways. If pid is 0, measurements
happen on the current thread, if pid is greater than 0, the process indicated by pid is measured, and if
pid is -1, all processes are counted.

The cpu argument allows measurements to be specific to a CPU. If cpu is greater than or equal to 0,
measurements are restricted to the specified CPU; if cpu is -1, the events are measured on all CPUs.

Note that the combination of pid == -1 and cpu == -1 is not valid.

A pid > 0 and cpu == -1 setting measures per-process and follows that process to whatever CPU the process
gets scheduled to. Per-process events can be created by any user.

A pid == -1 and cpu >= 0 setting is per-CPU and measures all processes on the specified CPU. Per-CPU
events need the CAP_SYS_ADMIN capability or a /proc/sys/kernel/perf_event_paranoid value of less than 1.

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 fd 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), etc., 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_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 re-routes 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 grain 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 sub-directory 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.

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.

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.

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.

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 recording of exec mmap events.

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, but enables including data mmap events in the ring-buffer.

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

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 bitflag.

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 bitmask 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 than 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;
__u64 cap_usr_time : 1,
cap_usr_rdpmc : 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 looks at 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 Add this to hardware counter value??

time_enabled
Time the event was active.

time_running
Time the event was running.

cap_usr_time
User time capability.

cap_usr_rdpmc
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;
s64 pmc = 0;

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;
pmc = rdpmc(idx - 1);
}

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

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;

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 over-write 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;
};

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[];
};

PERF_RECORD_EXIT
This record indicates a process exit event.

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

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;
};

PERF_RECORD_FORK
This record indicates a fork event.

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

PERF_RECORD_READ
This record indicates a read event.

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

PERF_RECORD_SAMPLE
This record indicates a sample.

struct {
struct perf_event_header header;
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 */
};

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 bitmask.

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

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 non-zero 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 non-zero, 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
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
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
IOC_PERIOD is the command to update the period; it does not update the current period but instead
defers until next.

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

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.

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 current 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 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 sub-directory contains information on the architecture-specific sub-fields 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 sub-directory contains files with pre-defined 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 than 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

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

ERRORS

       EINVAL Returned if the specified event is not available.

       ENOSPC Prior to Linux 3.3, if there was not enough room for the event, ENOSPC was  returned.   Linus  did
              not  like  this,  and  this  was  changed  to EINVAL.  ENOSPC is still returned if you try to read
              results into too small of a buffer.

VERSION

       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

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

NOTES

       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

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 non-zero 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.

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

       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>

       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);
       }

COLOPHON

       This  page  is  part  of  release 3.54 of the Linux man-pages project.  A description of the project, and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.