linux-kernel - Re: [Bug #12650] Strange load average and ksoftirqd behavior with 2.6.29-rc2-git1

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Date:	Sun, 15 Feb 2009 19:03:55 +0100
From:	Damien Wyart <damien.wyart@...e.fr>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>
Subject: Re: [Bug #12650] Strange load average and ksoftirqd behavior with
	2.6.29-rc2-git1

> > So I followed the tracing steps in the tutorial (with the 1 sec sleep),
> > which gave me this:
> > http://damien.wyart.free.fr/trace_2.6.29-rc5_ksoftirqd_prob.txt.gz

> thanks. There's definitely some weirdness visible in the trace,
> for example:

>   0)    gpm-1879    =>   ksoftir-4   
>   ------------------------------------------

>   0)   0.964 us    |    finish_task_switch();
>   0) ! 1768184 us |  }
>   0)               |  do_softirq() {
>   0)               |    __do_softirq() {
>   0)               |      rcu_process_callbacks() {

> the 1.7 seconds 'overhead' there must be a fluke - you'd notice it if
> ksoftirqd _really_ took that much time to execute.

> One possibility for these symptoms would be broken scheduler timestamps.
> Could you enable absolute timestamp printing via:

>     echo funcgraph-abstime  > trace_options

Mmm, seems I do not have this option recognized in rc5, so could not
test. Will retry all this with tip tomorrow...

> Also, my guess is that if you boot via idle=poll, the symptoms go away.
> This would strengthen the suspicion that it's scheduler-clock troubles.

In fact, with idle=poll, the symptoms do not go away, they are much
stronger: without it, ksotirqd have a few % of CPU in top output; with
it, they have 20 or 30% and the global average is not far from 1.

On my laptop (I do not have it at hand today), with rc3-gitX (did not
retest with rc5), the load avg was ok, but I saw that after boot,
ksoftird threads had a quite higher running time in top than with
2.6.28. I am surprised nobody reported this yet...

I attach to this mail config and dmesg if needed (this is rc5 without
idle=poll).

If a trace with funcgraph-abstime is interesting for you with tip,
I will do this tomorrow.

-- 
Damien Wyart

View attachment "config" of type "text/plain" (42926 bytes)

View attachment "dmesg" of type "text/plain" (23573 bytes)