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

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Message-ID: <20181120211803.GF13936@tassilo.jf.intel.com>
Date:   Tue, 20 Nov 2018 13:18:03 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Kyle Huey <me@...ehuey.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

> I suppose that's fair that it's better for some use cases. The flip
> side is that it's no longer possible to get exactly accurate counts
> from user space if you're using the PMI (because any events between
> the overflow itself and the transition to the PMI handler are
> permanently lost) which is catastrophically bad for us :)

Yes that's a fair point. For most usages it doesn't matter.

I suspect that's a case for supporting opt-out for freezing
per perf event, and rr using that.

-Andi