linux-kernel - Re: [PATCH] trace: reset sleep/block start time on task switch

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Date:	Thu, 26 Jan 2012 11:13:42 -0800
From:	Arun Sharma <asharma@...com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	<linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Andrew Vagin <avagin@...nvz.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] trace: reset sleep/block start time on task switch

On 1/25/12 6:27 PM, Frederic Weisbecker wrote:

>> Even if we resolve the sampling rate related problems, there is the
>> issue of: can we trust that a sampled sched_switch event and a
>> sampled sched_stat_sleep event actually match each other?
>
> Well, a sched_stat_sleep event should match the sched_switch with
> prev as the last targeted task.
>
> Or am I missing something?

I was thinking about large systems with 100k+ sched_switch events, where 
the user wants to just sample 1k events/sec.

perf record -F 1000 -e sched:sched_switch -a -- sleep 1

Doesn't look like we're using sampling for tracepoint events? The rest 
of this makes sense only if we're sampling tracepoint events (eg: to 
limit the impact on the system being profiled/traced).

On a system with 50k sched_switch and 10k sched_stat_sleep events, if we 
sample at 1000 events/sec, we may lose an event of interest either due 
to finite buffer sizes or to sampling, which is why a single event based 
sleep profiling is of interest to us.

On 1/25/12 6:21 PM, Frederic Weisbecker wrote:
 >
 > Have you tried to tweak the -m options to increase the size of the 
buffer?

-m4:

Processed 276875 events and lost 4772 chunks!

Check IO/CPU overload!

-m5:

   Fatal: failed to mmap with 22 (Invalid argument)

  -Arun
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