User: Password:
Subscribe / Log in / New account

perf: add support for sampling taken branches

From:  Stephane Eranian <>
Subject:  [PATCH v6 00/18] perf: add support for sampling taken branches
Date:  Thu, 9 Feb 2012 23:20:50 +0100
Message-ID:  <>
Archive-link:  Article

This patchset adds an important and useful new feature to
perf_events: branch stack sampling. In other words, the
ability to capture taken branches into each sample.

Statistical sampling of taken branch should not be confused
for branch tracing. Not all branches are necessarily captured

Sampling taken branches is important for basic block profiling,
statistical call graph, function call counts. Many of those
measurements can help drive a compiler optimizer.

The branch stack is a software abstraction which sits on top
of the PMU hardware. As such, it is not available on all
processors. For now, the patch provides the generic interface
and the Intel X86 implementation where it leverages the Last
Branch Record (LBR) feature (from Core2 to SandyBridge).

Branch stack sampling is supported for both per-thread and
system-wide modes.

It is possible to filter the type and privilege level of branches
to sample. The target of the branch is used to determine
the privilege level.

For each branch, the source and destination are captured. On
some hardware platforms, it may be possible to also extract
the target prediction and, in that case, it is also exposed
to end users.

The branch stack can record a variable number of taken
branches per sample. Those branches are always consecutive
in time. The number of branches captured depends on the
filtering and the underlying hardware. On Intel Nehalem
and later, up to 16 consecutive branches can be captured
per sample.

Branch sampling is always coupled with an event. It can
be any PMU event but it can't be a SW or tracepoint event.

Branch sampling is requested by setting a new sample_type

To support branch filtering, we introduce a new field
to the perf_event_attr struct: branch_sample_type. We chose
NOT to overload the config1, config2 field because those
are related to the event encoding. Branch stack is a
separate feature which is combined with the event.

The branch_sample_type is a bitmask of possible filters.
The following filters are defined (more can be added):
- PERF_SAMPLE_BRANCH_ANY     : any control flow change
- PERF_SAMPLE_BRANCH_USER    : branches when target is at user level
- PERF_SAMPLE_BRANCH_KERNEL  : branches when target is at kernel level
- PERF_SAMPLE_BRANCH_HV      : branches when target is at hypervisor level
- PERF_SAMPLE_BRANCH_ANY_CALL: call branches (incl. syscalls)
- PERF_SAMPLE_BRANCH_ANY_RET : return branches (incl. syscall returns)

It is possible to combine filters, e.g., IND_CALL|USER|KERNEL.

When the privilege level is not specified, the branch stack
inherits that of the associated event.

Some processors may not offer hardware branch filtering, e.g., Intel
Atom. Some may have HW filtering bugs (e.g., Nehalem). The Intel
X86 implementation in this patchset also provides a SW branch filter
which works on a best effort basis. It can compensate for the lack
of LBR filtering. But first and foremost, it helps work around LBR
filtering errata. The goal is to only capture the type of branches
requested by the user.

It is possible to combine branch stack sampling with PEBS on Intel
X86 processors. Depending on the precise_sampling mode, there are
certain filterting restrictions. When precise_sampling=1, then
there are no filtering restrictions. When precise_sampling > 1, 
then only ANY|USER|KERNEL filter can be used. This comes from
the fact that the kernel uses LBR to compensate for the PEBS
off-by-1 skid on the instruction pointer.

To demonstrate how the perf_event branch stack sampling interface
works, the patchset also modifies perf record to capture taken
branches. Similarly perf report is enhanced to display a histogram
of taken branches.

I would like to thank Roberto Vitillo @ LBL for his work on the perf
tool for this.

Enough talking, let's take a simple example. Our trivial test program
goes like this:

void f2(void)
void f3(void)
void f1(unsigned long n)
  if (n & 1UL)
int main(void)
  unsigned long i;

  for (i=0; i < N; i++)
  return 0;

$ perf record -b any branchy
$ perf report -b
# Events: 23K cycles
# Overhead  Source Symbol     Target Symbol
# ........  ................  ................

    18.13%  [.] f1            [.] main                          
    18.10%  [.] main          [.] main                          
    18.01%  [.] main          [.] f1                            
    15.69%  [.] f1            [.] f1                            
     9.11%  [.] f3            [.] f1                            
     6.78%  [.] f1            [.] f3                            
     6.74%  [.] f1            [.] f2                            
     6.71%  [.] f2            [.] f1                            

Of the total number of branches captured, 18.13% were from f1() -> main().

Let's make this clearer by filtering the user call branches only:

$ perf record -b any_call -e cycles:u branchy
$ perf report -b
# Events: 19K cycles
# Overhead  Source Symbol              Target Symbol
# ........  .........................  .........................
    52.50%  [.] main                   [.] f1                   
    23.99%  [.] f1                     [.] f3                   
    23.48%  [.] f1                     [.] f2                   
     0.03%  [.] _IO_default_xsputn     [.] _IO_new_file_overflow
     0.01%  [k] _start                 [k] __libc_start_main    

Now it is more obvious. %52 of all the captured branches where calls from main() -> f1().
The rest is split 50/50 between f1() -> f2() and f1() -> f3() which is expected given
that f1() dispatches based on odd vs. even values of n which is constantly increasing.

Here is a kernel example, where we want to sample indirect calls:
$ perf record -a -C 1 -b ind_call -e r1c4:k sleep 10 
$ perf report -b
# Overhead  Source Symbol               Target Symbol
# ........  ..........................  ..........................
    36.36%  [k] __delay                 [k] delay_tsc             
     9.09%  [k] ktime_get               [k] read_tsc              
     9.09%  [k] getnstimeofday          [k] read_tsc              
     9.09%  [k] notifier_call_chain     [k] tick_notify           
     4.55%  [k] cpuidle_idle_call       [k] intel_idle            
     4.55%  [k] cpuidle_idle_call       [k] menu_reflect          
     2.27%  [k] handle_irq              [k] handle_edge_irq       
     2.27%  [k] ack_apic_edge           [k] native_apic_mem_write 
     2.27%  [k] hpet_interrupt_handler  [k] hrtimer_interrupt     
     2.27%  [k] __run_hrtimer           [k] watchdog_timer_fn     
     2.27%  [k] enqueue_task            [k] enqueue_task_rt       
     2.27%  [k] try_to_wake_up          [k] select_task_rq_rt     
     2.27%  [k] do_timer                [k] read_tsc              

Due to HW limitations, branch filtering may be approximate on
Core, Atom processors. It is more accurate on Nehalem, Westmere
and best on Sandy Bridge.

In version 2, we've updated the patch to tip/master (commit 5734857) and
we've incoporated the feedback from v1 concerning anynous bitfield
struct for branch_stack_entry and the hanlding of i386 ABI binaries
on 64-bit host in the instr decoder for the LBR SW filter.

In version 3, we've updated to 3.2.0-tip. The Atom revision
check has been put into its own patch. We fixed a browser
issue with report report. We fixed all the style issues as well.

In version 4, we've modified the branch stack API to add a missing
priv level : hypervisor. There is a new PERF_SAMPLE_BRANCH_HV. It
is not used on Intel X86. Thanks to
for pointing this out. We also fix compilation error on ARM.

In version 4, we also extend the patch to include the changes necessary
to the perf tool to support reading files which were produced
from older perf_event ABI revisions. This patch set extends the ABI
with a new field in struct perf_event_attr. That struct is saved as
is in the file. Therefore, older files contain
smaller perf_event_attr struct, yet perf must process them transparently.
That's not the case today. It dies with 'incompatible file format'.

The patch solves this problem and, at the same time, decouples endianness
detection from the size of perf_event_attr. Endianness is now detected via
the signature (the first 8 bytes of the file). We introduce a new signature
(PERFILE2). It is not laid out the same way in the file based on the endianness
of the host where the file is written. Therefore, we can dynamically detect
the endianness by simply reading the first 8 bytes. The size of the
perf_event_attr struct can then be processed according to the endianness.
The ambiguity between the size being at the same time, the endianness marker
and the actual size is gone. We can now distinguish an older ABI by the size
and not confuse it with an endianness mismatch.

In version 5, we fix the PEBS+LBR vs. BRANCH_STACK check in x86_pmu_hw_config.
We also changed the handling of PERF_SAMPLE_BRANCH_HV on X86. It is now ignored
instead of triggering an error. That enables: perf record -b any -e cycles,
without having to force a priv level on the branch type. We also fix an
uninitialized variable bug in the perf tool reported by reviewers. Thanks
to Anshuman Khandual for his comments.

In version 6, we have fixed several issues in the perf tool code and
especially in patch 11. We have fully implemented the --sort option
on the branch source and target. We have fixed several column alignment
issues. We have integrated feedback from David Ahern, concerning patch
16 and the ability to read files written from a different
ABI. Perf will now reject any file that has a larger perf_event_attr
size. Perf provides compatibility backward and not forward.

Signed-off-by: Stephane Eranian <>

Roberto Agostino Vitillo (3):
  perf: add code to support PERF_SAMPLE_BRANCH_STACK
  perf: add support for sampling taken branch to perf record
  perf: add support for taken branch sampling to perf report

Stephane Eranian (15):
  perf: add generic taken branch sampling support
  perf: add Intel LBR MSR definitions
  perf: add Intel X86 LBR sharing logic
  perf: sync branch stack sampling with X86 precise_sampling
  perf: add Intel X86 LBR mappings for PERF_SAMPLE_BRANCH filters
  perf: disable LBR support for older Intel Atom processors
  perf: implement PERF_SAMPLE_BRANCH for Intel X86
  perf: add LBR software filter support for Intel X86
  perf: disable PERF_SAMPLE_BRANCH_* when not supported
  perf: add hook to flush branch_stack on context switch
  perf: fix endianness detection in
  perf: add ABI reference sizes
  perf: enable reading of files from different ABI rev
  perf: fix bug print_event_desc()
  perf: make perf able to read file from older ABIs

 arch/alpha/kernel/perf_event.c             |    4 +
 arch/arm/kernel/perf_event.c               |    4 +
 arch/mips/kernel/perf_event_mipsxx.c       |    4 +
 arch/powerpc/kernel/perf_event.c           |    4 +
 arch/sh/kernel/perf_event.c                |    4 +
 arch/sparc/kernel/perf_event.c             |    4 +
 arch/x86/include/asm/msr-index.h           |    7 +
 arch/x86/kernel/cpu/perf_event.c           |   85 ++++-
 arch/x86/kernel/cpu/perf_event.h           |   19 +
 arch/x86/kernel/cpu/perf_event_amd.c       |    3 +
 arch/x86/kernel/cpu/perf_event_intel.c     |  120 +++++--
 arch/x86/kernel/cpu/perf_event_intel_ds.c  |   22 +-
 arch/x86/kernel/cpu/perf_event_intel_lbr.c |  526 ++++++++++++++++++++++++++--
 include/linux/perf_event.h                 |   82 ++++-
 kernel/events/core.c                       |  177 ++++++++++
 kernel/events/hw_breakpoint.c              |    6 +
 tools/perf/Documentation/perf-record.txt   |   25 ++
 tools/perf/Documentation/perf-report.txt   |    7 +
 tools/perf/builtin-record.c                |   74 ++++
 tools/perf/builtin-report.c                |   98 +++++-
 tools/perf/perf.h                          |   18 +
 tools/perf/util/annotate.c                 |    2 +-
 tools/perf/util/event.h                    |    1 +
 tools/perf/util/evsel.c                    |   14 +
 tools/perf/util/header.c                   |  230 +++++++++++--
 tools/perf/util/hist.c                     |   93 ++++-
 tools/perf/util/hist.h                     |    7 +
 tools/perf/util/session.c                  |   72 ++++
 tools/perf/util/session.h                  |    4 +
 tools/perf/util/sort.c                     |  362 ++++++++++++++-----
 tools/perf/util/sort.h                     |    5 +
 tools/perf/util/symbol.h                   |   13 +
 32 files changed, 1866 insertions(+), 230 deletions(-)


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

Copyright © 2012, Eklektix, Inc.
Comments and public postings are copyrighted by their creators.
Linux is a registered trademark of Linus Torvalds