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

[Liang, Kan]

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,
> >   	 * of the count.
> >   	 */
> >   	delta = (new_raw_count << shift) - (prev_raw_count << shift);
> > +
> > +	/*
> > +	 * Take auto-reload into account
> > +	 * For the auto-reload before the last time, it went through the
> > +	 * whole period (reload_val) every time.
> > +	 * Just simply add period * times to the event.
> > +	 *
> > +	 * For the last load, the elapsed delta (event-)time need to be
> > +	 * corrected by adding the period. Because the start point is -period.
> > +	 */
> > +	if (reload_times > 0) {
> > +		delta += (reload_val << shift);
> > +		local64_add(reload_val * (reload_times - 1), &event->count);
> > +	}
> >   	delta >>= shift;
> >   
> >   	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,
> >   				   void *base, void *top,
> >   				   int bit, int count)
> >   {
> > +	struct hw_perf_event *hwc = &event->hw;
> >   	struct perf_sample_data data;
> >   	struct pt_regs regs;
> >   	void *at = get_next_pebs_record_by_bit(base, top, bit);
> >   
> > -	if (!intel_pmu_save_and_restart(event, 0, 0) &&
> > +	/*
> > +	 * Now, auto-reload is only enabled in fixed period mode.
> > +	 * The reload value is always hwc->sample_period.
> > +	 * May need to change it, if auto-reload is enabled in freq mode later.
> > +	 */
> > +	if (!intel_pmu_save_and_restart(event, hwc->sample_period, count - 1) &&
> >   	    !(event->hw.flags & PERF_X86_EVENT_AUTO_RELOAD))
> >   		return;
> >   
>
> 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.
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.

Thanks,
Kan