Messages in this thread | | | From | Alexey Budankov <> | Subject | [PATCH v3 0/4]: perf/core: fix restoring of Intel LBR call stack on a context switch | Date | Fri, 18 Oct 2019 12:17:42 +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 v3: - replaced assignment with swap at intel_pmu_lbr_sync_task_ctx()
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
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