Messages in this thread | | | From | kan.liang@linux ... | Subject | [PATCH 00/12] Stitch LBR call stack (Perf Tools) | Date | Fri, 28 Feb 2020 08:29:59 -0800 |
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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")
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
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