From: Kan Liang kan.liang@linux.intel.com
commit f861854e1b435b27197417f6f90d87188003cb24 upstream.
Perf doesn't take the left period into account when auto-reload is enabled with fixed period sampling mode in context switch.
Here is the MSR trace of the perf command as below. (The MSR trace is simplified from a ftrace log.)
#perf record -e cycles:p -c 2000000 -- ./triad_loop
//The MSR trace of task schedule out //perf disable all counters, disable PEBS, disable GP counter 0, //read GP counter 0, and re-enable all counters. //The counter 0 stops at 0xfffffff82840 write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value 0 write_msr: MSR_IA32_PEBS_ENABLE(3f1), value 0 write_msr: MSR_P6_EVNTSEL0(186), value 40003003c rdpmc: 0, value fffffff82840 write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value f000000ff
//The MSR trace of the same task schedule in again //perf disable all counters, enable and set GP counter 0, //enable PEBS, and re-enable all counters. //0xffffffe17b80 (-2000000) is written to GP counter 0. write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value 0 write_msr: MSR_IA32_PMC0(4c1), value ffffffe17b80 write_msr: MSR_P6_EVNTSEL0(186), value 40043003c write_msr: MSR_IA32_PEBS_ENABLE(3f1), value 1 write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value f000000ff
When the same task schedule in again, the counter should starts from previous left. However, it starts from the fixed period -2000000 again.
A special variant of intel_pmu_save_and_restart() is used for auto-reload, which doesn't update the hwc->period_left. When the monitored task schedules in again, perf doesn't know the left period. The fixed period is used, which is inaccurate.
With auto-reload, the counter always has a negative counter value. So the left period is -value. Update the period_left in intel_pmu_save_and_restart_reload().
With the patch:
//The MSR trace of task schedule out write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value 0 write_msr: MSR_IA32_PEBS_ENABLE(3f1), value 0 write_msr: MSR_P6_EVNTSEL0(186), value 40003003c rdpmc: 0, value ffffffe25cbc write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value f000000ff
//The MSR trace of the same task schedule in again write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value 0 write_msr: MSR_IA32_PMC0(4c1), value ffffffe25cbc write_msr: MSR_P6_EVNTSEL0(186), value 40043003c write_msr: MSR_IA32_PEBS_ENABLE(3f1), value 1 write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value f000000ff
Fixes: d31fc13fdcb2 ("perf/x86/intel: Fix event update for auto-reload") Signed-off-by: Kan Liang kan.liang@linux.intel.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Signed-off-by: Ingo Molnar mingo@kernel.org Link: https://lkml.kernel.org/r/20200121190125.3389-1-kan.liang@linux.intel.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/events/intel/ds.c | 2 ++ 1 file changed, 2 insertions(+)
--- a/arch/x86/events/intel/ds.c +++ b/arch/x86/events/intel/ds.c @@ -1368,6 +1368,8 @@ intel_pmu_save_and_restart_reload(struct old = ((s64)(prev_raw_count << shift) >> shift); local64_add(new - old + count * period, &event->count);
+ local64_set(&hwc->period_left, -new); + perf_event_update_userpage(event);
return 0;