Re: [PATCH 2/4] perf/x86/intel: fix event update for auto-reload

From: Liang, Kan
Date: Tue Dec 19 2017 - 15:24:57 EST

On 12/19/2017 3:08 PM, Liang, Kan wrote:

On 12/19/2017 1:58 PM, Peter Zijlstra wrote:
On Mon, Dec 18, 2017 at 03:34:49AM -0800, kan.liang@xxxxxxxxxxxxxxx wrote:
 arch/x86/events/core.c | 14 ++++++++++++++
 arch/x86/events/intel/ds.c | 8 +++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 35552ea..f74e21d 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -100,6 +100,20 @@ u64 x86_perf_event_update(struct perf_event *event,
ÂÂÂÂÂÂ * of the count.
ÂÂÂÂÂ delta = (new_raw_count << shift) - (prev_raw_count << shift);
+ÂÂÂ /*
+ÂÂÂÂ * Take auto-reload into account
+ÂÂÂÂ * For the auto-reload before the last time, it went through the
+ÂÂÂÂ * whole period (reload_val) every time.
+ÂÂÂÂ * Just simply add period * times to the event.
+ÂÂÂÂ * For the last load, the elapsed delta (event-)time need to be
+ÂÂÂÂ * corrected by adding the period. Because the start point is -period.
+ÂÂÂÂ */
+ÂÂÂ if (reload_times > 0) {
+ÂÂÂÂÂÂÂ delta += (reload_val << shift);
+ÂÂÂÂÂÂÂ local64_add(reload_val * (reload_times - 1), &event->count);
+ÂÂÂ }
ÂÂÂÂÂ delta >>= shift;
ÂÂÂÂÂ local64_add(delta, &event->count);
diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
index 0b693b7..f0f6026 100644
--- a/arch/x86/events/intel/ds.c
+++ b/arch/x86/events/intel/ds.c
@@ -1256,11 +1256,17 @@ static void __intel_pmu_pebs_event(struct perf_event *event,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ void *base, void *top,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ int bit, int count)
+ÂÂÂ struct hw_perf_event *hwc = &event->hw;
ÂÂÂÂÂ struct perf_sample_data data;
ÂÂÂÂÂ struct pt_regs regs;
ÂÂÂÂÂ void *at = get_next_pebs_record_by_bit(base, top, bit);
-ÂÂÂ if (!intel_pmu_save_and_restart(event, 0, 0) &&
+ÂÂÂ /*
+ÂÂÂÂ * Now, auto-reload is only enabled in fixed period mode.
+ÂÂÂÂ * The reload value is always hwc->sample_period.
+ÂÂÂÂ * May need to change it, if auto-reload is enabled in freq mode later.
+ÂÂÂÂ */
+ÂÂÂ if (!intel_pmu_save_and_restart(event, hwc->sample_period, count - 1) &&
ÂÂÂÂÂÂÂÂÂ !(event->hw.flags & PERF_X86_EVENT_AUTO_RELOAD))

This all looks very wrong... In auto reload we should never call
intel_pmu_save_and_restore() in the first place I think.

Things like x86_perf_event_update() and x86_perf_event_set_period()
simply _cannot_ do the right thing when we auto reload the counter.

I think it should be OK to call it in first place.
For x86_perf_event_update(), the reload_times will tell if it's auto reload. Both period_left and event->count are carefully recalculated for auto reload.

Think a bit more. You are right. We cannot rely on count to tell us if it's auto reload.
The count could also be 1 if auto reload is enabled.

I will fix it in V2.


For x86_perf_event_set_period(), there is nothing special needed for auto reload. The period is fixed. The period_left from x86_perf_event_update() is already handled.

BTW: It should be 'count' not 'count - 1' which pass to
intel_pmu_save_and_restart(). I just found the issue. I will fix it in
V2 with other improvements if there are any.