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

From: Peter Zijlstra
Date: Tue Dec 19 2017 - 17:07:23 EST

On Tue, Dec 19, 2017 at 03:08:58PM -0500, Liang, Kan wrote:
> > 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.

How does prev_count make sense when we've already reloaded a bunch of

> 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.

Hurm.. I see. But rather than make an ever bigger trainwreck of things,
I'd rather you just write a special purpose intel_pmu_save_and_restart()
just for AUTO_RELOAD.