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 11:13:51 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Damien Wyart <damien.wyart@...e.fr>
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:

> > > Load average is between 0.30 and 0.60 when the machine is idle, the
> > > two ksoftirqd threads get a very high total running time in top.
> > > This is on a P4 machine. On a recent laptop, the load is not as
> > > high, but the ksoftirqd threads also get a very high total running
> > > time.
> 
> > Mind having a look at this anomaly with the function graph tracer? We are
> > interested in a representative trace that shows the weird ksoftirq activities.
> > [ which trace could possibly pinpoint their origin. ]
> 
> > Here's the tracing quickstart:
> 
> >    http://redhat.com/~mingo/tip.git/tracing-quickstart.txt
> 
> > Also attached below. Let me know if you have trouble getting a good trace,
> > or if any of the steps were non-intuitive or burdensome to you.
> 
> Thanks for your feedback & explanations; I will try to have a look at
> all this material asap, I think tomorrow.

Note that if the box you test this on is multi-core or HT, then interpreting
traces is easier if there's just a single CPU to look at. In that case i'd 
suggest to reproduce with just a single core, by turning the second one off:

   echo 0 > /sys/devices/system/cpu/cpu1/online

Or, if the problem only occurs with two cpus, restrict tracing to CPU#1:

   echo 2 > /debug/tracing/tracing_cpumask 

	Ingo
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