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

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Message-ID: <20131226151429.GA15303@localhost.localdomain>
Date:	Thu, 26 Dec 2013 16:14:31 +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 Mon, Dec 23, 2013 at 09:44:25AM -0500, David Ahern wrote:
> On 12/23/13, 8:10 AM, Frederic Weisbecker wrote:
> >On Fri, Dec 20, 2013 at 10:09:53AM -0700, David Ahern wrote:
> >>On 12/20/13, 5:27 AM, Joseph Schuchart wrote:
> >>>I know this comes late, but: As far as I can see, your change does not
> >>>preserve the logic of the code I suggested. The idea was to first gather
> >>>all the maximum timestamps of all cpus (that is, the last timestamp seen
> >>>on each cpu) and then determine the minimum of these maxima. These are
> >>>two distinct steps that I think cannot be combined in one update. Your
> >>
> >>  A number of people have reported similar problems -- timestamps
> >>below last flush time. This approach would solve that problem for
> >>data processed from files, so it would be a good improvement.
> >
> >Could it be near what you're looking for?
> >
> >https://lkml.org/lkml/2012/2/18/53
> >
> 
> Forgot about that patch. It is similar to what Joseph wants for
> analyzing a file.
> 
> I was carrying that patch while working on perf-kvm-stat-live last
> Fall. It does not solve the problem for live commands, so ended up
> dropping it and going with local (to the command) hacks. I still
> think for live commands getting a perf_clock timestamp at the start
> of a round and using that as the flush time will work best.

Ok, but how would you fetch this perf clock timestamp, with an explicit read?

In the meantime, I can fix and post my old patch, which should solve at least
the perf.data based event stream.
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