linux-kernel - Re: [PATCH] Perf: Correct Assumptions about Sample Timestamps in Passes

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Date:	Thu, 9 Jan 2014 16:19:57 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	David Ahern <dsahern@...il.com>
Cc:	Joseph Schuchart <joseph.schuchart@...dresden.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	thomas.ilsche@...dresden.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Perf: Correct Assumptions about Sample Timestamps in
 Passes

On Wed, Jan 08, 2014 at 02:48:37PM -0700, David Ahern wrote:
> The existing code does not work. Your unstable tsc patch did not
> work. I have not tried Joseph's patch. Are you proposing that one or
> do you have something else in mind?

I think we should integrate Joseph's patch (or mine, or some mixup, I mean
they do about the same IIRC) as it solves known and understood bugs in any case.

Then we need to check what is the real issue in your case.

> 
> >Now there is still the problem of:
> >
> >1) local timestamps not moving forward (could it happen when events happen in storm,
> >when they overflow multiple times in once for example, and clock is not granular
> >enough?)
> 
> Even at 650k events/sec I am not seeing this problem.

Yeah it happens mostly when a single event, supposed to overflow on period of 1, trigger
with a higher period. This is the case of sched stat runtime tracepoints for example
because it is a weighted tracepoint (see perf_count). So it demux into gazillions of
events all having very close timestamps. But normal tracepoints shouldn't have this problem.

> 
> >Anyway this should be solved with the patch that takes the earliest last event on all
> >CPU buffer instead of the maximum of a round as a barrier.
> >
> >2) local timestamps not monotonic due to interrupting events. This could be fixed
> >in the kernel with moving perf_clock() snapshot in perf_output_sample().
> >
> 
> For perf-kvm the events are all tracepoints, so there should not be
> a problem of overlap due to interruption.

Nope, I'm curious what kind of issue happens with kvm events. Could you send me a perf.data
that has this ordering problem?

Thanks.
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