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Date: Wed, 4 Mar 2020 10:33:02 -0300
From: Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To: kan.liang@...ux.intel.com
Cc: jolsa@...hat.com, peterz@...radead.org, mingo@...hat.com,
linux-kernel@...r.kernel.org, namhyung@...nel.org,
adrian.hunter@...el.com, mathieu.poirier@...aro.org,
ravi.bangoria@...ux.ibm.com, alexey.budankov@...ux.intel.com,
vitaly.slobodskoy@...el.com, pavel.gerasimov@...el.com,
mpe@...erman.id.au, eranian@...gle.com, ak@...ux.intel.com
Subject: Re: [PATCH 00/12] Stitch LBR call stack (Perf Tools)
Em Fri, Feb 28, 2020 at 08:29:59AM -0800, kan.liang@...ux.intel.com escreveu:
> From: Kan Liang <kan.liang@...ux.intel.com>
>
> The kernel patches have been merged into linux-next.
> commit bbfd5e4fab63 ("perf/core: Add new branch sample type for HW
> index of raw branch records")
> commit db278b90c326 ("perf/x86/intel: Output LBR TOS information
> correctly")
I saw it landed in tip/perf/core, going thru this patchset now.
Thanks,
- Arnaldo
> Start from Haswell, Linux perf can utilize the existing Last Branch
> Record (LBR) facility to record call stack. However, the depth of the
> reconstructed LBR call stack limits to the number of LBR registers.
> E.g. on skylake, the depth of reconstructed LBR call stack is <= 32
> That's because HW will overwrite the oldest LBR registers when it's
> full.
>
> However, the overwritten LBRs may still be retrieved from previous
> sample. At that moment, HW hasn't overwritten the LBR registers yet.
> Perf tools can stitch those overwritten LBRs on current call stacks to
> get a more complete call stack.
>
> To determine if LBRs can be stitched, the physical index of LBR
> registers is required. A new branch sample type is introduced to dump
> the physical index of the most recent LBR aka Top-of-Stack (TOS)
> information for perf tools.
> Patch 1 & 2 extend struct branch_stack to support the new branch sample
> type, PERF_SAMPLE_BRANCH_HW_INDEX.
>
> Since the output format of PERF_SAMPLE_BRANCH_STACK will be changed
> when the new branch sample type is set, an older version of perf tool
> may parse the perf.data incorrectly. Furthermore, there is no warning
> if this case happens. Because current perf header never check for
> unknown input bits in attr. Patch 3 adds check for event attr. (Can be
> merged independently.)
>
> Besides the physical index, the maximum number of LBRs is required as
> well. Patch 4 & 5 retrieve the capabilities information from sysfs
> and save them in perf header.
>
> Patch 6 & 7 implements the LBR stitching approach.
>
> Users can use the options introduced in patch 8-11 to enable the LBR
> stitching approach for perf report, script, top and c2c.
>
> Patch 12 adds a fast path for duplicate entries check. It benefits all
> call stack parsing, not just for stitch LBR call stack. It can be
> merged independently.
>
>
> The stitching approach base on LBR call stack technology. The known
> limitations of LBR call stack technology still apply to the approach,
> e.g. Exception handing such as setjmp/longjmp will have calls/returns
> not match.
> This approach is not full proof. There can be cases where it creates
> incorrect call stacks from incorrect matches. There is no attempt
> to validate any matches in another way. So it is not enabled by default.
> However in many common cases with call stack overflows it can recreate
> better call stacks than the default lbr call stack output. So if there
> are problems with LBR overflows this is a possible workaround.
>
> Regression:
> Users may collect LBR call stack on a machine with new perf tool and
> new kernel (support LBR TOS). However, they may parse the perf.data with
> old perf tool (not support LBR TOS). The old tool doesn't check
> attr.branch_sample_type. Users probably get incorrect information
> without any warning.
>
> Performance impact:
> The processing time may increase with the LBR stitching approach
> enabled. The impact depends on the increased depth of call stacks.
>
> For a simple test case tchain_edit with 43 depth of call stacks.
> perf record --call-graph lbr -- ./tchain_edit
> perf report --stitch-lbr
>
> Without --stitch-lbr, perf report only display 32 depth of call stacks.
> With --stitch-lbr, perf report can display all 43 depth of call stacks.
> The depth of call stacks increase 34.3%.
>
> Correspondingly, the processing time of perf report increases 39%,
> Without --stitch-lbr: 11.0 sec
> With --stitch-lbr: 15.3 sec
>
> The source code of tchain_edit.c is something similar as below.
> noinline void f43(void)
> {
> int i;
> for (i = 0; i < 10000;) {
>
> if(i%2)
> i++;
> else
> i++;
> }
> }
>
> noinline void f42(void)
> {
> int i;
> for (i = 0; i < 100; i++) {
> f43();
> f43();
> f43();
> }
> }
>
> noinline void f41(void)
> {
> int i;
> for (i = 0; i < 100; i++) {
> f42();
> f42();
> f42();
> }
> }
> noinline void f40(void)
> {
> f41();
> }
>
> ... ...
>
> noinline void f32(void)
> {
> f33();
> }
>
> noinline void f31(void)
> {
> int i;
>
> for (i = 0; i < 10000; i++) {
> if(i%2)
> i++;
> else
> i++;
> }
>
> f32();
> }
>
> noinline void f30(void)
> {
> f31();
> }
>
> ... ...
>
> noinline void f1(void)
> {
> f2();
> }
>
> int main()
> {
> f1();
> }
>
> Kan Liang (12):
> perf tools: Add hw_idx in struct branch_stack
> perf tools: Support PERF_SAMPLE_BRANCH_HW_INDEX
> perf header: Add check for event attr
> perf pmu: Add support for PMU capabilities
> perf header: Support CPU PMU capabilities
> perf machine: Refine the function for LBR call stack reconstruction
> perf tools: Stitch LBR call stack
> perf report: Add option to enable the LBR stitching approach
> perf script: Add option to enable the LBR stitching approach
> perf top: Add option to enable the LBR stitching approach
> perf c2c: Add option to enable the LBR stitching approach
> perf hist: Add fast path for duplicate entries check approach
>
> tools/include/uapi/linux/perf_event.h | 8 +-
> tools/perf/Documentation/perf-c2c.txt | 11 +
> tools/perf/Documentation/perf-report.txt | 11 +
> tools/perf/Documentation/perf-script.txt | 11 +
> tools/perf/Documentation/perf-top.txt | 9 +
> .../Documentation/perf.data-file-format.txt | 16 +
> tools/perf/builtin-c2c.c | 6 +
> tools/perf/builtin-record.c | 3 +
> tools/perf/builtin-report.c | 6 +
> tools/perf/builtin-script.c | 76 ++--
> tools/perf/builtin-stat.c | 1 +
> tools/perf/builtin-top.c | 11 +
> tools/perf/tests/sample-parsing.c | 7 +-
> tools/perf/util/branch.h | 27 +-
> tools/perf/util/callchain.h | 12 +-
> tools/perf/util/cs-etm.c | 1 +
> tools/perf/util/env.h | 3 +
> tools/perf/util/event.h | 1 +
> tools/perf/util/evsel.c | 20 +-
> tools/perf/util/evsel.h | 6 +
> tools/perf/util/header.c | 147 ++++++
> tools/perf/util/header.h | 1 +
> tools/perf/util/hist.c | 26 +-
> tools/perf/util/intel-pt.c | 2 +
> tools/perf/util/machine.c | 424 +++++++++++++++---
> tools/perf/util/perf_event_attr_fprintf.c | 1 +
> tools/perf/util/pmu.c | 87 ++++
> tools/perf/util/pmu.h | 12 +
> .../scripting-engines/trace-event-python.c | 30 +-
> tools/perf/util/session.c | 8 +-
> tools/perf/util/sort.c | 2 +-
> tools/perf/util/sort.h | 2 +
> tools/perf/util/synthetic-events.c | 6 +-
> tools/perf/util/thread.c | 2 +
> tools/perf/util/thread.h | 34 ++
> tools/perf/util/top.h | 1 +
> 36 files changed, 900 insertions(+), 131 deletions(-)
>
> --
> 2.17.1
>
--
- Arnaldo
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