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Date:   Tue, 20 Nov 2018 13:18:03 -0800
From:   Andi Kleen <ak@linux.intel.com>
To:     Kyle Huey <me@kylehuey.com>
Cc:     Kan Liang <kan.liang@linux.intel.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
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        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Stephane Eranian <eranian@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vince Weaver <vincent.weaver@maine.edu>, acme@kernel.org,
        open list <linux-kernel@vger.kernel.org>
Subject: Re: [REGRESSION] x86, perf: counter freezing breaks rr
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> 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