[PATCH] perf/x86/intel: Fix inaccurate period in context switch for auto-reload

[kan . liang]

From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>

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 ftrace when recording PEBS event with fixed period.

    #perf record -e cycles:p -c 2000000 -- ./triad_loop

      //Task is scheduled out
      triad_loop-17222 [000] d... 861765.878032: write_msr:
MSR_CORE_PERF_GLOBAL_CTRL(38f), value 0  //Disable global counter
      triad_loop-17222 [000] d... 861765.878033: write_msr:
MSR_IA32_PEBS_ENABLE(3f1), value 0       //Disable PEBS
      triad_loop-17222 [000] d... 861765.878033: write_msr:
MSR_P6_EVNTSEL0(186), value 40003003c    //Disable the counter
      triad_loop-17222 [000] d... 861765.878033: rdpmc: 0, value
fffffff82840                             //Read value of the counter
      triad_loop-17222 [000] d... 861765.878034: write_msr:
MSR_CORE_PERF_GLOBAL_CTRL(38f), value 1000f000000ff  //Re-enable global
counter

      //Task is scheduled in again
      triad_loop-17222 [000] d... 861765.878221: write_msr:
MSR_CORE_PERF_GLOBAL_CTRL(38f), value 0  //Disable global counter
      triad_loop-17222 [000] d... 861765.878222: write_msr:
MSR_IA32_PMC0(4c1), value ffffffe17b80   //write the value to the
counter; The value is wrong. When the task switch in again, the counter
should starts from previous left. However, it starts from the fixed
period -2000000 again.
      triad_loop-17222 [000] d... 861765.878223: write_msr:
MSR_P6_EVNTSEL0(186), value 40043003c    //enable the counter
      triad_loop-17222 [000] d... 861765.878223: write_msr:
MSR_IA32_PEBS_ENABLE(3f1), value 1       //enable PEBS
      triad_loop-17222 [000] d... 861765.878223: write_msr:
MSR_CORE_PERF_GLOBAL_CTRL(38f), value 1000f000000ff  //Re-enable global
counter

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 scheduled in again, perf doesn't know the left
period. The user defined 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,
      //Task is scheduled out
      triad_loop-3068  [000] d...   153.680459: write_msr:
MSR_CORE_PERF_GLOBAL_CTRL(38f), value 0
      triad_loop-3068  [000] d...   153.680459: write_msr:
MSR_IA32_PEBS_ENABLE(3f1), value 0
      triad_loop-3068  [000] d...   153.680459: write_msr:
MSR_P6_EVNTSEL0(186), value 40003003c
      triad_loop-3068  [000] d...   153.680459: rdpmc: 0, value
ffffffe25cbc
      triad_loop-3068  [000] d...   153.680460: write_msr:
MSR_CORE_PERF_GLOBAL_CTRL(38f), value f000000ff

      //Task is scheduled in again
      triad_loop-3068  [000] d...   153.680644: write_msr:
MSR_CORE_PERF_GLOBAL_CTRL(38f), value 0
      triad_loop-3068  [000] d...   153.680646: write_msr:
MSR_IA32_PMC0(4c1), value ffffffe25cbc     //The left value is written
into the counter.
      triad_loop-3068  [000] d...   153.680646: write_msr:
MSR_P6_EVNTSEL0(186), value 40043003c
      triad_loop-3068  [000] d...   153.680646: write_msr:
MSR_IA32_PEBS_ENABLE(3f1), value 1
      triad_loop-3068  [000] d...   153.680647: 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@xxxxxxxxxxxxxxx>
---
 arch/x86/events/intel/ds.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
index ce83950036c5..e5ad97a82342 100644
--- a/arch/x86/events/intel/ds.c
+++ b/arch/x86/events/intel/ds.c
@@ -1713,6 +1713,8 @@ intel_pmu_save_and_restart_reload(struct perf_event *event, int count)
 	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;
-- 
2.17.1