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

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Date:   Tue, 19 Dec 2017 14:11:39 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Liang, Kan" <kan.liang@...ux.intel.com>, mingo@...hat.com,
        acme@...nel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de,
        jolsa@...hat.com, eranian@...gle.com
Subject: Re: [PATCH 2/4] perf/x86/intel: fix event update for auto-reload

On Tue, Dec 19, 2017 at 11:07:09PM +0100, Peter Zijlstra wrote:
> 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?

We can figure it out how often there was a reload based on the PEBS index.

-Andi