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

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Date:   Tue, 19 Dec 2017 23:07:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     mingo@...hat.com, acme@...nel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, jolsa@...hat.com, eranian@...gle.com,
        ak@...ux.intel.com
Subject: Re: [PATCH 2/4] perf/x86/intel: fix event update for auto-reload

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
times?

> 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.