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

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Message-ID: <20090215110104.GB31351@elte.hu>
Date:	Sun, 15 Feb 2009 12:01:04 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Damien Wyart <damien.wyart@...e.fr>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>,
	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	"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


* Damien Wyart <damien.wyart@...e.fr> wrote:

> 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

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.

	Ingo
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