linux-kernel - Re: [REGRESSION] x86, perf: counter freezing breaks rr

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Date:   Tue, 27 Nov 2018 14:08:46 -0800
From:   Kyle Huey <me@...ehuey.com>
To:     "Peter Zijlstra (Intel)" <peterz@...radead.org>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Robert O'Callahan" <robert@...llahan.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Stephane Eranian <eranian@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vince Weaver <vincent.weaver@...ne.edu>, acme@...nel.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [REGRESSION] x86, perf: counter freezing breaks rr

On Wed, Nov 21, 2018 at 12:14 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Nov 20, 2018 at 02:38:54PM -0800, Andi Kleen wrote:
> > > In fact, I'll argue FREEZE_ON_OVERFLOW is unfixably broken for
> > > independent counters, because while one counter overflows, we'll stall
> > > counting on all others until we've handled the PMI.
> > >
> > > Even though the PMI might not be for them and they very much want/need
> > > to continue counting.
> >
> > We stop all counters in any case for the PMI. With freeze-on-PMI it just
> > happens slightly earlier.
>
> Hiding the PMI is fine and good. The PMI is not the workload. Stopping
> it earlier is _NOT_ good, it hides your actual workload.

It does seem that FREEZE_PERFMON_ON_PMI (misnamed as it is) is of
rather limited use (or even negative, in our case) to a counter that's
already restricted to ring 3.

- Kyle