linux-kernel - Re: [PATCH 3/6] perf: add reference time event

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Date:	Tue, 12 Jul 2011 19:03:20 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	David Ahern <dsahern@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	paulus@...ba.org, tglx@...utronix.de
Subject: Re: [PATCH 3/6] perf: add reference time event

On Tue, Jul 12, 2011 at 10:35:21AM -0600, David Ahern wrote:
> On 07/12/2011 08:30 AM, Frederic Weisbecker wrote:
> > Actually you first have to face a deeper problem. events are not stored
> > in order in the flow, but they are sorted from perf_session__process_events().
> > 
> > The bunch of sorted events is flushed periodically and sent to the consumer.
> > 
> > See flush_sample_queue().
> > 
> > And this sorting is made on top of the sample->time timestamps. So events
> > are first sorted on sample->time and only afterward you have access to your
> > gtod tracepoint samples. But if that gtod sample has been taken after a reboot
> > then its sample->time is not consistant with the rest. It is not well sorted
> > and thus the reftime won't be updated at the right moment.
> 
> I was aware of this function and the buffering /sorting it does; I have
> similar code in my local analysis tool (for older perf userspace). I had
> not stopped to consider the impacts on the append mode since I never use
> that option.
> 
> > So the problem is that reftime update already depends on a consistant cpu
> > timestamp.
> > 
> > I can't think about a sane way to work around that. Sorting on gtod + cpu timestamp
> > is not a solution because gtod can change.
> > 
> > I'd rather propose to refuse append mode as long as we have any timestamp. That includes
> > gtod but also sample timestamps. They are buggy if we reboot.
> 
> I'm fine with restricting append mode if samples have timestamps. In
> that case the time of day reference can be added to the header per the
> original patch set. The change in behavior does impact a lot of record
> use cases (system wide, raw).

Yep, sounds good.
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