path: root/Documentation/userspace-api/perf_ring_buffer.rst

.. SPDX-License-Identifier: GPL-2.0

================
Perf ring buffer
================

.. CONTENTS

    1. Introduction

    2. Ring buffer implementation
    2.1  Basic algorithm
    2.2  Ring buffer for different tracing modes
    2.2.1       Default mode
    2.2.2       Per-thread mode
    2.2.3       Per-CPU mode
    2.2.4       System wide mode
    2.3  Accessing buffer
    2.3.1       Producer-consumer model
    2.3.2       Properties of the ring buffers
    2.3.3       Writing samples into buffer
    2.3.4       Reading samples from buffer
    2.3.5       Memory synchronization

    3. The mechanism of AUX ring buffer
    3.1  The relationship between AUX and regular ring buffers
    3.2  AUX events
    3.3  Snapshot mode


1. Introduction
===============

The ring buffer is a fundamental mechanism for data transfer.  perf uses
ring buffers to transfer event data from kernel to user space, another
kind of ring buffer which is so called auxiliary (AUX) ring buffer also
plays an important role for hardware tracing with Intel PT, Arm
CoreSight, etc.

The ring buffer implementation is critical but it's also a very
challenging work.  On the one hand, the kernel and perf tool in the user
space use the ring buffer to exchange data and stores data into data
file, thus the ring buffer needs to transfer data with high throughput;
on the other hand, the ring buffer management should avoid significant
overload to distract profiling results.

This documentation dives into the details for perf ring buffer with two
parts: firstly it explains the perf ring buffer implementation, then the
second part discusses the AUX ring buffer mechanism.

2. Ring buffer implementation
=============================

2.1 Basic algorithm
-------------------

That said, a typical ring buffer is managed by a head pointer and a tail
pointer; the head pointer is manipulated by a writer and the tail
pointer is updated by a reader respectively.

::

        +---------------------------+
        |   |   |***|***|***|   |   |
        +---------------------------+
                `-> Tail    `-> Head

        * : the data is filled by the writer.

                Figure 1. Ring buffer

Perf uses the same way to manage its ring buffer.  In the implementation
there are two key data structures held together in a set of consecutive
pages, the control structure and then the ring buffer itself.  The page
with the control structure in is known as the "user page".  Being held
in continuous virtual addresses simplifies locating the ring buffer
address, it is in the pages after the page with the user page.

The control structure is named as ``perf_event_mmap_page``, it contains a
head pointer ``data_head`` and a tail pointer ``data_tail``.  When the
kernel starts to fill records into the ring buffer, it updates the head
pointer to reserve the memory so later it can safely store events into
the buffer.  On the other side, when the user page is a writable mapping,
the perf tool has the permission to update the tail pointer after consuming
data from the ring buffer.  Yet another case is for the user page's
read-only mapping, which is to be addressed in the section
:ref:`writing_samples_into_buffer`.

::

          user page                          ring buffer
    +---------+---------+   +---------------------------------------+
    |data_head|data_tail|...|   |   |***|***|***|***|***|   |   |   |
    +---------+---------+   +---------------------------------------+
        `          `----------------^                   ^
         `----------------------------------------------|

              * : the data is filled by the writer.

                Figure 2. Perf ring buffer

When using the ``perf record`` tool, we can specify the ring buffer size
with option ``-m`` or ``--mmap-pages=``, the given size will be rounded up
to a power of two that is a multiple of a page size.  Though the kernel
allocates at once for all memory pages, it's deferred to map the pages
to VMA area until the perf tool accesses the buffer from the user space.
In other words, at the first time accesses the buffer's page from user
space in the perf tool, a data abort exception for page fault is taken
and the kernel uses this occasion to map the page into process VMA
(see ``perf_mmap_fault()``), thus the perf tool can continue to access
the page after returning from the exception.

2.2 Ring buffer for different tracing modes
-------------------------------------------

The perf profiles programs with different modes: default mode, per thread
mode, per cpu mode, and system wide mode.  This section describes these
modes and how the ring buffer meets requirements for them.  At last we
will review the race conditions caused by these modes.

2.2.1 Default mode
^^^^^^^^^^^^^^^^^^

Usually we execute ``perf record`` command followed by a profiling program
name, like below command::

        perf record test_program

This command doesn't specify any options for CPU and thread modes, the
perf tool applies the default mode on the perf event.  It maps all the
CPUs in the system and the profiled program's PID on the perf event, and
it enables inheritance mode on the event so that child tasks inherits
the events.  As a result, the perf event is attributed as::

    evsel::cpus::map[]    = { 0 .. _SC_NPROCESSORS_ONLN-1 }
    evsel::threads::map[] = { pid }
    evsel::attr::inherit  = 1

These attributions finally will be reflected on the deployment of ring
buffers.  As shown below, the perf tool allocates individual ring buffer
for each CPU, but it only enables events for the profiled program rather
than for all threads in the system.  The *T1* thread represents the
thread context of the 'test_program', whereas *T2* and *T3* are irrelevant
threads in the system.   The perf samples are exclusively collected for
the *T1* thread and stored in the ring buffer associated with the CPU on
which the *T1* thread is running.

::

              T1                      T2                 T1
            +----+              +-----------+          +----+
    CPU0    |xxxx|              |xxxxxxxxxxx|          |xxxx|
            +----+--------------+-----------+----------+----+-------->
              |                                          |
              v                                          v
            +-----------------------------------------------------+
            |                  Ring buffer 0                      |
            +-----------------------------------------------------+

                   T1
                 +-----+
    CPU1         |xxxxx|
            -----+-----+--------------------------------------------->
                    |
                    v
            +-----------------------------------------------------+
            |                  Ring buffer 1                      |
            +-----------------------------------------------------+

                                        T1              T3
                                      +----+        +-------+
    CPU2                              |xxxx|        |xxxxxxx|
            --------------------------+----+--------+-------+-------->
                                        |
                                        v
            +-----------------------------------------------------+
            |                  Ring buffer 2                      |
            +-----------------------------------------------------+

                              T1
                       +--------------+
    CPU3               |xxxxxxxxxxxxxx|
            -----------+--------------+------------------------------>
                              |
                              v
            +-----------------------------------------------------+
            |                  Ring buffer 3                      |
            +-----------------------------------------------------+

	    T1: Thread 1; T2: Thread 2; T3: Thread 3
	    x: Thread is in running state

                Figure 3. Ring buffer for default mode

2.2.2 Per-thread mode
^^^^^^^^^^^^^^^^^^^^^

By specifying option ``--per-thread`` in perf command, e.g.

::

        perf record --per-thread test_program

The perf event doesn't map to any CPUs and is only bound to the
profiled process, thus, the perf event's attributions are::

    evsel::cpus::map[0]   = { -1 }
    evsel::threads::map[] = { pid }
    evsel::attr::inherit  = 0

In this mode, a single ring buffer is allocated for the profiled thread;
if the thread is scheduled on a CPU, the events on that CPU will be
enabled; and if the thread is scheduled out from the CPU, the events on
the CPU will be disabled.  When the thread is migrated from one CPU to
another, the events are to be disabled on the previous CPU and enabled
on the next CPU correspondingly.

::

              T1                      T2                 T1
            +----+              +-----------+          +----+
    CPU0    |xxxx|              |xxxxxxxxxxx|          |xxxx|
            +----+--------------+-----------+----------+----+-------->
              |                                           |
              |    T1                                     |
              |  +-----+                                  |
    CPU1      |  |xxxxx|                                  |
            --|--+-----+----------------------------------|---------->
              |     |                                     |
              |     |                   T1            T3  |
              |     |                 +----+        +---+ |
    CPU2      |     |                 |xxxx|        |xxx| |
            --|-----|-----------------+----+--------+---+-|---------->
              |     |                   |                 |
              |     |         T1        |                 |
              |     |  +--------------+ |                 |
    CPU3      |     |  |xxxxxxxxxxxxxx| |                 |
            --|-----|--+--------------+-|-----------------|---------->
              |     |         |         |                 |
              v     v         v         v                 v
            +-----------------------------------------------------+
            |                  Ring buffer                        |
            +-----------------------------------------------------+

            T1: Thread 1
            x: Thread is in running state

                Figure 4. Ring buffer for per-thread mode

When perf runs in per-thread mode, a ring buffer is allocated for the
profiled thread *T1*.  The ring buffer is dedicated for thread *T1*, if the
thread *T1* is running, the perf events will be recorded into the ring
buffer; when the thread is sleeping, all associated events will be
disabled, thus no trace data will be recorded into the ring buffer.

2.2.3 Per-CPU mode
^^^^^^^^^^^^^^^^^^

The option ``-C`` is used to collect samples on the list of CPUs, for
example the below perf command receives option ``-C 0,2``::

	perf record -C 0,2 test_program

It maps the perf event to CPUs 0 and 2, and the event is not associated to any
PID.  Thus the perf event attributions are set as::

    evsel::cpus::map[0]   = { 0, 2 }
    evsel::threads::map[] = { -1 }
    evsel::attr::inherit  = 0

This results in the session of ``perf record`` will sample all threads on CPU0
and CPU2, and be terminated until test_program exits.  Even there have tasks
running on CPU1 and CPU3, since the ring buffer is absent for them, any
activities on these two CPUs will be ignored.  A usage case is to combine the
options for per-thread mode and per-CPU mode, e.g. the options ``–C 0,2`` and
``––per–thread`` are specified together, the samples are recorded only when
the profiled thread is scheduled on any of the listed CPUs.

::

              T1                      T2                 T1
            +----+              +-----------+          +----+
    CPU0    |xxxx|              |xxxxxxxxxxx|          |xxxx|
            +----+--------------+-----------+----------+----+-------->
              |                       |                  |
              v                       v                  v
            +-----------------------------------------------------+
            |                  Ring buffer 0                      |
            +-----------------------------------------------------+

                   T1
                 +-----+
    CPU1         |xxxxx|
            -----+-----+--------------------------------------------->

                                        T1              T3
                                      +----+        +-------+
    CPU2                              |xxxx|        |xxxxxxx|
            --------------------------+----+--------+-------+-------->
                                        |               |
                                        v               v
            +-----------------------------------------------------+
            |                  Ring buffer 1                      |
            +-----------------------------------------------------+

                              T1
                       +--------------+
    CPU3               |xxxxxxxxxxxxxx|
            -----------+--------------+------------------------------>

            T1: Thread 1; T2: Thread 2; T3: Thread 3
            x: Thread is in running state

                Figure 5. Ring buffer for per-CPU mode

2.2.4 System wide mode
^^^^^^^^^^^^^^^^^^^^^^

By using option ``–a`` or ``––all–cpus``, perf collects samples on all CPUs
for all tasks, we call it as the system wide mode, the command is::

        perf record -a test_program

Similar to the per-CPU mode, the perf event doesn't bind to any PID, and
it maps to all CPUs in the system::

   evsel::cpus::map[]    = { 0 .. _SC_NPROCESSORS_ONLN-1 }
   evsel::threads::map[] = { -1 }
   evsel::attr::inherit  = 0

In the system wide mode, every CPU has its own ring buffer, all threads
are monitored during the running state and the samples are recorded into
the ring buffer belonging to the CPU which the events occurred on.

::

              T1                      T2                 T1
            +----+              +-----------+          +----+
    CPU0    |xxxx|              |xxxxxxxxxxx|          |xxxx|
            +----+--------------+-----------+----------+----+-------->
              |                       |                  |
              v                       v                  v
            +-----------------------------------------------------+
            |                  Ring buffer 0                      |
            +-----------------------------------------------------+

                   T1
                 +-----+
    CPU1         |xxxxx|
            -----+-----+--------------------------------------------->
                    |
                    v
            +-----------------------------------------------------+
            |                  Ring buffer 1                      |
            +-----------------------------------------------------+

                                        T1              T3
                                      +----+        +-------+
    CPU2                              |xxxx|        |xxxxxxx|
            --------------------------+----+--------+-------+-------->
                                        |               |
                                        v               v
            +-----------------------------------------------------+
            |                  Ring buffer 2                      |
            +-----------------------------------------------------+

                              T1
                       +--------------+
    CPU3               |xxxxxxxxxxxxxx|
            -----------+--------------+------------------------------>
                              |
                              v
            +-----------------------------------------------------+
            |                  Ring buffer 3                      |
            +-----------------------------------------------------+

            T1: Thread 1; T2: Thread 2; T3: Thread 3
            x: Thread is in running state

                Figure 6. Ring buffer for system wide mode

2.3 Accessing buffer
--------------------

Based on the understanding of how the ring buffer is allocated in
various modes, this section explains access the ring buffer.

2.3.1 Producer-consumer model
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In the Linux kernel, the PMU events can produce samples which are stored
into the ring buffer; the perf command in user space consumes the
samples by reading out data from the ring buffer and finally saves the
data into the file for post analysis.  It’s a typical producer-consumer
model for using the ring buffer.

The perf process polls on the PMU events and sleeps when no events are
incoming.  To prevent frequent exchanges between the kernel and user
space, the kernel event core layer introduces a watermark, which is
stored in the ``perf_buffer::watermark``.  When a sample is recorded into
the ring buffer, and if the used buffer exceeds the watermark, the
kernel wakes up the perf process to read samples from the ring buffer.

::

                       Perf
                       / | Read samples
             Polling  /  `--------------|               Ring buffer
                     v                  v    ;---------------------v
    +----------------+     +---------+---------+   +-------------------+
    |Event wait queue|     |data_head|data_tail|   |***|***|   |   |***|
    +----------------+     +---------+---------+   +-------------------+
             ^                  ^ `------------------------^
             | Wake up tasks    | Store samples
          +-----------------------------+
          |  Kernel event core layer    |
          +-----------------------------+

              * : the data is filled by the writer.

                Figure 7. Writing and reading the ring buffer

When the kernel event core layer notifies the user space, because
multiple events might share the same ring buffer for recording samples,
the core layer iterates every event associated with the ring buffer and
wakes up tasks waiting on the event.  This is fulfilled by the kernel
function ``ring_buffer_wakeup()``.

After the perf process is woken up, it starts to check the ring buffers
one by one, if it finds any ring buffer containing samples it will read
out the samples for statistics or saving into the data file.  Given the
perf process is able to run on any CPU, this leads to the ring buffer
potentially being accessed from multiple CPUs simultaneously, which
causes race conditions.  The race condition handling is described in the
section :ref:`memory_synchronization`.

2.3.2 Properties of the ring buffers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Linux kernel supports two write directions for the ring buffer: forward and
backward.  The forward writing saves samples from the beginning of the ring
buffer, the backward writing stores data from the end of the ring buffer with
the reversed direction.  The perf tool determines the writing direction.

Additionally, the tool can map buffers in either read-write mode or read-only
mode to the user space.

The ring buffer in the read-write mode is mapped with the property
``PROT_READ | PROT_WRITE``.  With the write permission, the perf tool
updates the ``data_tail`` to indicate the data start position.  Combining
with the head pointer ``data_head``, which works as the end position of
the current data, the perf tool can easily know where read out the data
from.

Alternatively, in the read-only mode, only the kernel keeps to update
the ``data_head`` while the user space cannot access the ``data_tail`` due
to the mapping property ``PROT_READ``.

As a result, the matrix below illustrates the various combinations of
direction and mapping characteristics.  The perf tool employs two of these
combinations to support buffer types: the non-overwrite buffer and the
overwritable buffer.

.. list-table::
   :widths: 1 1 1
   :header-rows: 1

   * - Mapping mode
     - Forward
     - Backward
   * - read-write
     - Non-overwrite ring buffer
     - Not used
   * - read-only
     - Not used
     - Overwritable ring buffer

The non-overwrite ring buffer uses the read-write mapping with forward
writing.  It starts to save data from the beginning of the ring buffer
and wrap around when overflow, which is used with the read-write mode in
the normal ring buffer.  When the consumer doesn't keep up with the
producer, it would lose some data, the kernel keeps how many records it
lost and generates the ``PERF_RECORD_LOST`` records in the next time
when it finds a space in the ring buffer.

The overwritable ring buffer uses the backward writing with the
read-only mode.  It saves the data from the end of the ring buffer and
the ``data_head`` keeps the position of current data, the perf always
knows where it starts to read and until the end of the ring buffer, thus
it don't need the ``data_tail``.  In this mode, it will not generate the
``PERF_RECORD_LOST`` records.

.. _writing_samples_into_buffer:

2.3.3 Writing samples into buffer
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When a sample is taken and saved into the ring buffer, the kernel
prepares sample fields based on the sample type; then it prepares the
info for writing ring buffer which is stored in the structure
``perf_output_handle``.  In the end, the kernel outputs the sample into
the ring buffer and updates the head pointer in the user page so the
perf tool can see the latest value.

The structure ``perf_output_handle`` serves as a temporary context for
tracking the information related to the buffer.  The advantages of it is
that it enables concurrent writing to the buffer by different events.
For example, a software event and a hardware PMU event both are enabled
for profiling, two instances of ``perf_output_handle`` serve as separate
contexts for the software event and the hardware event respectively.
This allows each event to reserve its own memory space for populating
the record data.

2.3.4 Reading samples from buffer
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In the user space, the perf tool utilizes the ``perf_event_mmap_page``
structure to handle the head and tail of the buffer.  It also uses
``perf_mmap`` structure to keep track of a context for the ring buffer, this
context includes information about the buffer's starting and ending
addresses.  Additionally, the mask value can be utilized to compute the
circular buffer pointer even for an overflow.

Similar to the kernel, the perf tool in the user space first reads out
the recorded data from the ring buffer, and then updates the buffer's
tail pointer ``perf_event_mmap_page::data_tail``.

.. _memory_synchronization:

2.3.5 Memory synchronization
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The modern CPUs with relaxed memory model cannot promise the memory
ordering, this means it’s possible to access the ring buffer and the
``perf_event_mmap_page`` structure out of order.  To assure the specific
sequence for memory accessing perf ring buffer, memory barriers are
used to assure the data dependency.  The rationale for the memory
synchronization is as below::

  Kernel                          User space

  if (LOAD ->data_tail) {         LOAD ->data_head
                   (A)            smp_rmb()        (C)
    STORE $data                   LOAD $data
    smp_wmb()      (B)            smp_mb()         (D)
    STORE ->data_head             STORE ->data_tail
  }

The comments in tools/include/linux/ring_buffer.h gives nice description
for w