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Subject[PATCH v2 0/4]: perf/core: fix restoring of Intel LBR call stack on a context switch
Date

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

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