Messages in this thread | | | From | Arnaldo Carvalho de Melo <> | Date | Wed, 4 Mar 2020 10:33:02 -0300 | Subject | Re: [PATCH 00/12] Stitch LBR call stack (Perf Tools) |
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Em Fri, Feb 28, 2020 at 08:29:59AM -0800, kan.liang@linux.intel.com escreveu: > From: Kan Liang <kan.liang@linux.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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