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

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Date:	Wed, 01 Jan 2014 11:37:55 -0700
From:	David Ahern <dsahern@...il.com>
To:	Frederic Weisbecker <fweisbec@...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 12/26/13, 8:30 AM, Frederic Weisbecker wrote:
> On Thu, Dec 26, 2013 at 10:24:03AM -0500, David Ahern wrote:
>> On 12/26/13, 10:14 AM, Frederic Weisbecker wrote:
>>>> 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.

For perf-kvm-stat-live using perf_clock value at the start of the round 
as the flush time works beautifully:

https://github.com/dsahern/linux/commit/ba8b7b63d5dbdc95aedbbafa670c2232e0cc07a2

Never once failed with "Warning: Timestamp below last timeslice flush" 
error.

David

>>>
>>> Ok, but how would you fetch this perf clock timestamp, with an explicit read?
>>
>> One option is exporting perf_clock to userspace.
>>
>> Preferably a built-in option, https://lkml.org/lkml/2013/12/10/781,
>> but I can go the module route if I have to:
>> https://github.com/dsahern/linux/blob/perf-full-monty/README.ahern
>
> I see, that's one of the very verbose threads I've seen lately ;)
>


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