Manpages

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.

use_clockid (since Linux 4.1)

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

context_switch (since Linux 4.3)

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

wakeup_events, wakeup_watermark

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

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

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

bp_type (since Linux 2.6.33)

This chooses the breakpoint type. It is one of:
HW_BREAKPOINT_EMPTY

No breakpoint.

HW_BREAKPOINT_R

Count when we read the memory location.

HW_BREAKPOINT_W

Count when we write the memory location.

HW_BREAKPOINT_RW

Count when we read or write the memory location.

HW_BREAKPOINT_X

Count when we execute code at the memory location.

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

bp_addr (since Linux 2.6.33)

This is the address of the breakpoint. For execution breakpoints, this is the memory address of the instruction of interest; for read and write breakpoints, it is the memory address of the memory location of interest.

config1 (since Linux 2.6.39)

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

bp_len (since Linux 2.6.33)

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

config2 (since Linux 2.6.39)

config2 is a further extension of the config1 field.

branch_sample_type (since Linux 3.4)

If PERF_SAMPLE_BRANCH_STACK is enabled, then this specifies what branches to include in the branch record.

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

Branch target is in user space.

PERF_SAMPLE_BRANCH_KERNEL

Branch target is in kernel space.

PERF_SAMPLE_BRANCH_HV

Branch target is in hypervisor.

PERF_SAMPLE_BRANCH_PLM_ALL

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

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

Any branch type.

PERF_SAMPLE_BRANCH_ANY_CALL

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

PERF_SAMPLE_BRANCH_IND_CALL

Indirect calls.

PERF_SAMPLE_BRANCH_CALL (since Linux 4.4)

Direct calls.

PERF_SAMPLE_BRANCH_ANY_RETURN

Any return branch.

PERF_SAMPLE_BRANCH_IND_JUMP (since Linux 4.2)

Indirect jumps.

PERF_SAMPLE_BRANCH_COND (since Linux 3.16)

Conditional branches.

PERF_SAMPLE_BRANCH_ABORT_TX (since Linux 3.11)

Transactional memory aborts.

PERF_SAMPLE_BRANCH_IN_TX (since Linux 3.11)

Branch in transactional memory transaction.

PERF_SAMPLE_BRANCH_NO_TX (since Linux 3.11)

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

sample_regs_user (since Linux 3.7)

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

sample_stack_user (since Linux 3.7)

This defines the size of the user stack to dump if PERF_SAMPLE_STACK_USER is specified.

clockid (since Linux 4.1)

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

aux_watermark (since Linux 4.1)

This specifies how much data is required to trigger a PERF_RECORD_AUX sample.

sample_max_stack (since Linux 4.8)

When sample_type includes PERF_SAMPLE_CALLCHAIN, this field specifies how many stack frames to report when generating the callchain.

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

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

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

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];
};

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:

time_enabled, time_running

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

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

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

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

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

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

}

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

Version number of this structure.

compat_version

The lowest version this is compatible with.

time_enabled

Time the event was active.

time_running

Time the event was running.

cap_usr_time / cap_usr_rdpmc / cap_bit0 (since Linux 3.4)

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

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

cap_bit0_is_deprecated (since Linux 3.12)

If set, this bit indicates that the kernel supports the properly separated cap_user_time and cap_user_rdpmc bits.

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

cap_user_rdpmc (since Linux 3.12)

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

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

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

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

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

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

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

cap_user_time (since Linux 3.12)

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

cap_user_time_zero (since Linux 3.12)

Indicates the presence of time_zero which allows mapping timestamp values to the hardware clock.

pmc_width

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

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

time_shift, time_mult, time_offset

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

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

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

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

time_zero (since Linux 3.12)

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

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

And vice versa:

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

data_head

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

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

data_tail

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

data_offset (since Linux 4.1)

Contains the offset of the location in the mmap buffer where perf sample data begins.

data_size (since Linux 4.1)

Contains the size of the perf sample region within the mmap buffer.

aux_head, aux_tail, aux_offset, aux_size (since Linux 4.1)

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

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

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

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

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

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

The mmap values start with a header:

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

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

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

Unknown CPU mode.

PERF_RECORD_MISC_KERNEL

Sample happened in the kernel.

PERF_RECORD_MISC_USER

Sample happened in user code.

PERF_RECORD_MISC_HYPERVISOR

Sample happened in the hypervisor.

PERF_RECORD_MISC_GUEST_KERNEL (since Linux 2.6.35)

Sample happened in the guest kernel.

PERF_RECORD_MISC_GUEST_USER (since Linux 2.6.35)

Sample happened in guest user code.

Since the following three statuses are generated by different record types, they alias to the same bit:
PERF_RECORD_MISC_MMAP_DATA (since Linux 3.10)

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

PERF_RECORD_MISC_COMM_EXEC (since Linux 3.16)

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

PERF_RECORD_MISC_SWITCH_OUT (since Linux 4.3)

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

In addition, the following bits can be set:
PERF_RECORD_MISC_EXACT_IP

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

PERF_RECORD_MISC_EXT_RESERVED (since Linux 2.6.35)

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

PERF_RECORD_MISC_PROC_MAP_PARSE_TIMEOUT

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

PERF_RECORD_MMAP

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

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

PERF_RECORD_LOST

This record indicates when events are lost.

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

PERF_RECORD_COMM

This record indicates a change in the process name.

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

PERF_RECORD_EXIT

This record indicates a process exit event.

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

PERF_RECORD_THROTTLE, PERF_RECORD_UNTHROTTLE

This record indicates a throttle/unthrottle event.

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

PERF_RECORD_FORK

This record indicates a fork event.

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

PERF_RECORD_READ

This record indicates a read event.

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

PERF_RECORD_SAMPLE

This record indicates a sample.

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

sample_id

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

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.

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.

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:

mispred

The branch target was mispredicted.

predicted

The branch target was predicted.

in_tx (since Linux 3.11)

The branch was in a transactional memory transaction.

abort (since Linux 3.11)

The branch was in an aborted transactional memory transaction.

cycles (since Linux 4.3)

This reports the number of cycles elapsed since the previous branch stack update.

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

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

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

abi, regs[weight(mask)]

If PERF_SAMPLE_REGS_USER is enabled, then the user CPU registers are recorded.

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

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

size, data[size], dyn_size

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

weight

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

data_src

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

Type of opcode, a bitwise combination of:

mem_lvl

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

mem_snoop

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

mem_lock

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

mem_dtlb

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

transaction

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

The field is a bitwise combination of the following values:
PERF_TXN_ELISION

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

PERF_TXN_TRANSACTION

Abort from a generic transaction.

PERF_TXN_SYNC

Synchronous abort (related to the reported instruction).

PERF_TXN_ASYNC

Asynchronous abort (not related to the reported instruction).

PERF_TXN_RETRY

Retryable abort (retrying the transaction may have succeeded).

PERF_TXN_CONFLICT

Abort due to memory conflicts with other threads.

PERF_TXN_CAPACITY_WRITE

Abort due to write capacity overflow.

PERF_TXN_CAPACITY_READ

Abort due to read capacity overflow.

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

abi, regs[weight(mask)]

If PERF_SAMPLE_REGS_INTR is enabled, then the user CPU registers are recorded.

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

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

PERF_RECORD_MMAP2

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

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

ino_generation

is the inode generation.

filename

is a string describing the backing of the allocated memory.

PERF_RECORD_AUX (since Linux 4.1)

This record reports that new data is available in the separate AUX buffer region.

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

aux_offset

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

aux_size

size of the data made available.

PERF_AUX_FLAG_TRUNCATED

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

PERF_AUX_FLAG_OVERWRITE

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

PERF_RECORD_ITRACE_START (since Linux 4.1)

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

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

PERF_RECORD_LOST_SAMPLES (since Linux 4.2)

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

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

PERF_RECORD_SWITCH (since Linux 4.3)

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

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

PERF_RECORD_SWITCH_CPU_WIDE (since Linux 4.3)

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

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

next_prev_pid

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

next_prev_tid

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

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

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

There are two ways to generate overflow notifications.

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

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

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

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

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

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

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

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

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

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

PERF_EVENT_IOC_DISABLE

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

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

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

PERF_EVENT_IOC_REFRESH

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

PERF_EVENT_IOC_RESET

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

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

PERF_EVENT_IOC_PERIOD

This updates the overflow period for the event.

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

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

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

PERF_EVENT_IOC_SET_OUTPUT

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

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

PERF_EVENT_IOC_SET_FILTER (since Linux 2.6.33)

This adds an ftrace filter to this event.

The argument is a pointer to the desired ftrace filter.

PERF_EVENT_IOC_ID (since Linux 3.12)

This returns the event ID value for the given event file descriptor.

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

PERF_EVENT_IOC_SET_BPF (since Linux 4.1)

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

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

PERF_EVENT_IOC_PAUSE_OUTPUT (since Linux 4.7)

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

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

PERF_EVENT_MODIFY_ATTRIBUTES (since Linux 4.17)

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

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

PERF_EVENT_IOC_QUERY_BPF (since Linux 4.16)

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

The argument is a pointer to a structure

struct perf_event_query_bpf {
    __u32    ids_len;
    __u32    prog_cnt;
    __u32    ids[0];
};

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

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

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

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

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

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

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

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

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

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

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

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

    close(fd);
}