[PATCH 00/12] perf_events: add support for sampling taken branches
From: Stephane Eranian
Date: Thu Oct 06 2011 - 10:49:40 EST
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
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
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
flag called: PERF_SAMPLE_BRANCH_STACK.
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 : capture branches when target is at user level
- PERF_SAMPLE_BRANCH_KERNEL : capture branches when target is at user level
- PERF_SAMPLE_BRANCH_ANY_CALL: capture call branches (incl. syscalls)
- PERF_SAMPLE_BRANCH_ANY_RET : capture return branches (incl. syscall returns)
- PERF_SAMPLE_BRANCH_IND_CALL: capture indirect calls
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 f1(unsigned long n)
if (n & 1UL)
unsigned long i;
for (i=0; i < N; i++)
$ 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
# 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.
Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
Roberto Agostino Vitillo (2):
perf: add support for sampling taken branch to perf record
perf: add support for taken branch sampling to perf report
Stephane Eranian (10):
perf_events: add generic taken branch sampling support
perf_events: add Intel LBR MSR definitions
perf_events: add Intel X86 LBR sharing logic
perf_events: sync branch stack sampling with X86 precise_sampling
perf_events: add LBR mappings for PERF_SAMPLE_BRANCH filters
perf_events: implement PERF_SAMPLE_BRANCH for Intel X86
perf_events: add LBR software filter support for Intel X86
perf_events: disable PERF_SAMPLE_BRANCH_* when not supported
perf_events: add hook to flush branch_stack on context switch
perf: add code to support PERF_SAMPLE_BRANCH_STACK
arch/alpha/kernel/perf_event.c | 4 +
arch/arm/kernel/perf_event.c | 4 +
arch/mips/kernel/perf_event.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 | 62 +++-
arch/x86/kernel/cpu/perf_event_amd.c | 3 +
arch/x86/kernel/cpu/perf_event_intel.c | 126 +++++--
arch/x86/kernel/cpu/perf_event_intel_ds.c | 21 +-
arch/x86/kernel/cpu/perf_event_intel_lbr.c | 529 ++++++++++++++++++++++++++--
include/linux/perf_event.h | 74 ++++-
kernel/events/core.c | 167 +++++++++
kernel/events/hw_breakpoint.c | 6 +
tools/perf/Documentation/perf-record.txt | 18 +
tools/perf/Documentation/perf-report.txt | 7 +
tools/perf/builtin-record.c | 75 ++++
tools/perf/builtin-report.c | 93 +++++-
tools/perf/perf.h | 17 +
tools/perf/util/annotate.c | 2 +-
tools/perf/util/event.h | 1 +
tools/perf/util/evsel.c | 10 +
tools/perf/util/hist.c | 97 ++++--
tools/perf/util/hist.h | 6 +
tools/perf/util/session.c | 72 ++++
tools/perf/util/session.h | 5 +
tools/perf/util/sort.c | 348 ++++++++++++++-----
tools/perf/util/sort.h | 5 +
tools/perf/util/symbol.h | 13 +
30 files changed, 1584 insertions(+), 204 deletions(-)
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