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

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Message-ID: <CAP045ApoZamGdgVd=-MJE52rUU5hnOg=j95B1WV2x_bVtRQpWA@mail.gmail.com>
Date:   Tue, 20 Nov 2018 11:54:56 -0800
From:   Kyle Huey <me@...ehuey.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Kan Liang <kan.liang@...ux.intel.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        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 Tue, Nov 20, 2018 at 11:41 AM Andi Kleen <ak@...ux.intel.com> wrote:
>
> > rr, a userspace record and replay debugger[0], uses the PMU interrupt
> > (PMI) to stop a program during replay to inject asynchronous events
> > such as signals. With perf counter freezing enabled we are reliably
> > seeing perf event overcounts during replay. This behavior is easily
>
> It's hard to see how it could over count since the PMU freezes
> earlier than the PMI with freezing. So it should count less.
> Did you mean under count?

Yes, I did mean under count, see my last email.

> > Given that we're already at rc3, and that this renders rr unusable,
> > we'd ask that counter freezing be disabled for the 4.20 release.
>
> The boot option should be good enough for the release?

I'm not entirely sure what you mean here. We want you to flip the
default boot option so this feature is off for this release. i.e. rr
should work by default on 4.20 and people should have to opt into the
inaccurate behavior if they want faster PMI servicing.

> A reasonable future option would be to expose an option to disable it in
> the perf_event. Then rr could set it.

- Kyle