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Date: Fri, 17 Apr 2020 17:47:49 -0400
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Arnaldo Carvalho de Melo <arnaldo.melo@...il.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 V4 00/17] Stitch LBR call stack (Perf Tools)
On 4/17/2020 1:48 PM, Arnaldo Carvalho de Melo wrote:
> Em Thu, Mar 19, 2020 at 01:25:00PM -0700, kan.liang@...ux.intel.com escreveu:
>> From: Kan Liang <kan.liang@...ux.intel.com>
>>
>> Changes since V3:
>> - There is no dependency among the 'capabilities'. If perf fails to read
>> one, it should not impact others. Continue to parse the rest of caps.
>> (Patch 1)
>> - Use list_for_each_entry() to replace perf_pmu__scan_caps() (Patch 1 &
>> 2)
>> - Combine the declaration plus assignment when possible (Patch 1 & 2)
>> - Add check for script/report/c2c.. (Patch 13, 14 & 16)
>>
>> Changes since V2:
>> - Check strdup() in Patch 1
>> - Split several patches into smaller patches
>>
>> Changes since V1:
>> - Rebase on top of commit 5100c2b77049 ("perf header: Add check for
>> unexpected use of reserved membrs in event attr")
>> - Fix compling error with GCC9 in patch 1.
>>
>>
>> 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")
>>
>> 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 maximum number of LBRs is
>> required. Patch 1 - 4 retrieve the capabilities information from sysfs
>> and save them in perf header.
>>
>> Patch 5 - 12 implements the LBR stitching approach.
>>
>> Users can use the options introduced in patch 13-16 to enable the LBR
>> stitching approach for perf report, script, top and c2c.
>>
>> Patch 17 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
>
> Next time provide the full test proggie, I had to expand those ... to
> reproduce your results,
Sure, I will do so in the future.
> all I have is in perf/core, some patches are
> still to be processed, will continue later, have to stop now, see:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/commit/?id=13cfba6b741ff
>
> For my testing, looks really great!
Thanks for the testing. :)
Kan
>
> - Arnaldo
>
>> 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 (17):
>> perf pmu: Add support for PMU capabilities
>> perf header: Support CPU PMU capabilities
>> perf record: Clear HEADER_CPU_PMU_CAPS for non LBR call stack mode
>> perf stat: Clear HEADER_CPU_PMU_CAPS
>> perf machine: Remove the indent in resolve_lbr_callchain_sample
>> perf machine: Refine the function for LBR call stack reconstruction
>> perf machine: Factor out lbr_callchain_add_kernel_ip()
>> perf machine: Factor out lbr_callchain_add_lbr_ip()
>> perf thread: Add a knob for LBR stitch approach
>> perf tools: Save previous sample for LBR stitching approach
>> perf tools: Save previous cursor nodes for LBR stitching approach
>> 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
>>
>> 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 | 12 +
>> tools/perf/builtin-record.c | 3 +
>> tools/perf/builtin-report.c | 12 +
>> tools/perf/builtin-script.c | 12 +
>> tools/perf/builtin-stat.c | 1 +
>> tools/perf/builtin-top.c | 11 +
>> tools/perf/util/branch.h | 19 +-
>> tools/perf/util/callchain.h | 8 +
>> tools/perf/util/env.h | 3 +
>> tools/perf/util/header.c | 108 +++++
>> tools/perf/util/header.h | 1 +
>> tools/perf/util/hist.c | 23 +
>> tools/perf/util/machine.c | 423 +++++++++++++++---
>> tools/perf/util/pmu.c | 82 ++++
>> tools/perf/util/pmu.h | 9 +
>> tools/perf/util/sort.c | 2 +-
>> tools/perf/util/sort.h | 2 +
>> tools/perf/util/thread.c | 2 +
>> tools/perf/util/thread.h | 35 ++
>> tools/perf/util/top.h | 1 +
>> 25 files changed, 757 insertions(+), 70 deletions(-)
>>
>> --
>> 2.17.1
>>
>
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