[PATCH V2] 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 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@xxxxxxxxxxxxxxx>
---

Changes since V1
- Update description

 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