linux-kernel - Re: How how latent should non-preemptive scheduling be?

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Date:	Sat, 27 Sep 2008 22:48:34 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Sitsofe Wheeler <sitsofe@...oo.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arjan van de Ven <arjan@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: How how latent should non-preemptive scheduling be?


* Sitsofe Wheeler <sitsofe@...oo.com> wrote:

> Logs with the filtering on can be seen here (15Mbytes decompressed each):
> http://sucs.org/~sits/test/eeepc-debug/20080923/latency_trace.gz
> http://sucs.org/~sits/test/eeepc-debug/20080923/trace.txt.gz
>
> It looks like lots of acpi state is created and deleted...

yeah. The latency starts here:

     cat-5901    0dNh.    1us : default_wake_function (__wake_up_common)
     cat-5901    0.Nh.    2us : kill_fasync (snd_pcm_period_elapsed)
     [...]

and ends here:

     [...]
     cat-5901    0.N.. 270501us+: mutex_lock (acpi_ec_transaction)
     cat-5901    0d... 270507us : __cond_resched (_cond_resched)

270 _milliseconds_ later. That's excessive.

The main overhead is starting here:

     cat-5901    0.N..  167us : acpi_ds_result_push (acpi_ds_exec_end_op)

lots of ACPI code executed ...

does it get better if you have CONFIG_PREEMPT_VOLUNTARY=y enabled? That 
_should_ break up this section neatly. If it doesnt then please add a 
might_sleep() check to kernel/kernel/semaphore.c's down_timeout() 
function - that is called a number of times in this trace.

	Ingo
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