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From:   Alexey Budankov <alexey.budankov@linux.intel.com>
Subject: [PATCH v2 0/4]: perf/core: fix restoring of Intel LBR call stack on a
 context switch
Organization: Intel Corp.
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Stephane Eranian <eranian@google.com>,
        Ian Rogers <irogers@google.com>,
        Song Liu <songliubraving@fb.com>,
        linux-kernel <linux-kernel@vger.kernel.org>
Message-ID: <5964c7e9-ab6f-c0d0-3dca-31196606e337@linux.intel.com>
Date:   Wed, 16 Oct 2019 12:41:43 +0300
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Restore Intel LBR call stack from cloned inactive task perf context on
a context switch. This change inherently addresses inconsistency in LBR 
call stack data provided on a sample in record profiling mode:

  $ perf record -N -B -T -R --call-graph lbr \
         -e cpu/period=0xcdfe60,event=0x3c,name=\'CPU_CLK_UNHALTED.THREAD\'/Duk \
         --clockid=monotonic_raw -- ./miniFE.x nx 25 ny 25 nz 25

Let's assume threads A, B, C belonging to the same process. 
B and C are siblings of A and their perf contexts are treated as equivalent.
At some point B blocks on a futex (non preempt context switch).
B's LBRs are preserved at B's perf context task_ctx_data and B's events 
are removed from PMU and disabled. B's perf context becomes inactive.

Later C gets on a cpu, runs, gets profiled and eventually switches to 
the awaken but not yet running B. The optimized context switch path is 
executed swapping B's and C's task_ctx_data pointers at perf event contexts.
So C's task_ctx_data will refer preserved B's LBRs on the following 
switch-in event.

However, as far B's perf context is inactive there is no enabled events
in there and B's task_ctx_data->lbr_callstack_users is equal to 0.
When B gets on the cpu B's events reviving is skipped following
the optimized context switch path and B's task_ctx_data->lbr_callstack_users
remains 0. Thus B's LBR's are not restored by pmu sched_task() code called 
in the end of perf context switch-in callback for B.

In the report that manifests as having short fragments of B's
call stack, still tracked by LBR's HW between adjacent samples,
but the whole thread call tree doesn't aggregate.

The fix has been evaluated when profiling miniFE [1] (C++, OpenMP)
workload running 64 threads on Intel Skylake EP(64 core, 2 sockets):

  $ perf report --call-graph callee,flat

5.3.0-rc6+ (tip perf/core) - fixed

-   92.66%    82.64%  miniFE.x  libiomp5.so         [.] _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_release
   - 69.14% _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_release
        __kmp_fork_barrier
        __kmp_launch_thread
        _INTERNAL_24_______src_z_Linux_util_c_3e0095e6::__kmp_launch_worker
        start_thread
        __clone
   - 21.89% _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_release
        __kmp_barrier
        __kmpc_reduce_nowait
        miniFE::cg_solve<miniFE::CSRMatrix<double, int, int>, miniFE::Vector<double, int, int>, miniFE::matvec_std<miniFE::CSRMatrix<double, int, int>, miniFE::Vector<double, int, in
        __kmp_invoke_microtask
        __kmp_invoke_task_func
        __kmp_launch_thread
        _INTERNAL_24_______src_z_Linux_util_c_3e0095e6::__kmp_launch_worker
        start_thread
        __clone
   - 1.63% _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_release
        __kmp_barrier
        __kmpc_reduce_nowait
        main
        __kmp_invoke_microtask
        __kmp_invoke_task_func
        __kmp_launch_thread
        _INTERNAL_24_______src_z_Linux_util_c_3e0095e6::__kmp_launch_worker
        start_thread
        __clone

5.0.13-300.fc30.x86_64 - no fix

-   90.29%    81.01%  miniFE.x  libiomp5.so         [.] _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_release
   - 33.45% _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_release
        __kmp_fork_barrier
        __kmp_launch_thread
        _INTERNAL_24_______src_z_Linux_util_c_3e0095e6::__kmp_launch_worker
        start_thread
        __clone
     87.63% _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_release
   - 54.79% _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_release
        __kmp_fork_barrier
        __kmp_launch_thread
   - 9.18% _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_release
        __kmp_barrier
        __kmpc_reduce_nowait
        miniFE::cg_solve<miniFE::CSRMatrix<double, int, int>, miniFE::Vector<double, int, int>, miniFE::matvec_std<miniFE::CSRMatrix<double, int, int>, miniFE::Vector<double, int, in
        __kmp_invoke_microtask
        __kmp_invoke_task_func
        __kmp_launch_thread
        _INTERNAL_24_______src_z_Linux_util_c_3e0095e6::__kmp_launch_worker
        start_thread
        __clone
   - 41.28% _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_release
        __kmp_fork_barrier
        __kmp_launch_thread
        _INTERNAL_24_______src_z_Linux_util_c_3e0095e6::__kmp_launch_worker
   - 15.77% _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_release
        __kmp_barrier
        __kmpc_reduce_nowait
        miniFE::cg_solve<miniFE::CSRMatrix<double, int, int>, miniFE::Vector<double, int, int>, miniFE::matvec_std<miniFE::CSRMatrix<double, int, int>, miniFE::Vector<double, int, in
        __kmp_invoke_microtask
        __kmp_invoke_task_func
        __kmp_launch_thread
   - 11.56% _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_release
        __kmp_barrier
        __kmpc_reduce_nowait
        miniFE::cg_solve<miniFE::CSRMatrix<double, int, int>, miniFE::Vector<double, int, int>, miniFE::matvec_std<miniFE::CSRMatrix<double, int, int>, miniFE::Vector<double, int, in
        __kmp_invoke_microtask
        __kmp_invoke_task_func
        __kmp_launch_thread
        _INTERNAL_24_______src_z_Linux_util_c_3e0095e6::__kmp_launch_worker
   - 2.33% _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_release
        __kmp_barrier
        __kmpc_reduce_nowait
        main
        __kmp_invoke_microtask
        __kmp_invoke_task_func
        __kmp_launch_thread
        _INTERNAL_24_______src_z_Linux_util_c_3e0095e6::__kmp_launch_worker
        start_thread
        __clone
     0.67% _INTERNAL_25_______src_kmp_barrier_cpp_1d20fae8::__kmp_hyper_barrier_gather
     0.57% __kmp_hardware_timestamp

[1] https://www.hpcadvisorycouncil.com/pdf/miniFE_Analysis_and_Profiling.pdf

---
Alexey Budankov (4):
  perf/core,x86: introduce sync_task_ctx() method at struct pmu
  perf/x86: install platform specific sync_task_ctx adapter
  perf/x86/intel: implement LBR callstacks context synchronization
  perf/core,x86: synchronize PMU task contexts on optimized context
    switches

 arch/x86/events/core.c       |  7 +++++++
 arch/x86/events/intel/core.c |  7 +++++++
 arch/x86/events/intel/lbr.c  |  9 +++++++++
 arch/x86/events/perf_event.h | 11 +++++++++++
 include/linux/perf_event.h   |  7 +++++++
 kernel/events/core.c         |  9 +++++++++
 6 files changed, 50 insertions(+)

---
Changes in v2:
- implemented sync_task_ctx() method at perf,x86,intel pmu types;
- employed the method on the optimized context switch path between 
  equivalent perf event contexts;

-- 
2.20.1