linux-kernel - Re: Dynticks Causing High Context Switch Rate in ksoftirqd

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Message-Id: <20071127212217.a7ac0407.akpm@linux-foundation.org>
Date:	Tue, 27 Nov 2007 21:22:17 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	bdupree@...hfinesse.com
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Dynticks Causing High Context Switch Rate in ksoftirqd

On Mon, 26 Nov 2007 20:36:32 -0600 (CST) bdupree@...hfinesse.com wrote:

> Question: Why is ksoftirqd eating about 5 to 10 percent of my CPU on an idle
> system? The problem occurs if I config the kernel with tickless
> support (i.e. CONFIG_TICK_ONESHOT=y).  (Thanks to "oprofile" for putting me
> onto this.)

beware that oprofile can provide misleading results on a paritally-idle
system.  You may have discovered that ksoftirqd is consuming 5-10% of the
non-idle time on that idle system, which is less surprising.

> I have noted this same problem on kernel versions: 2.6.23.1, 2.6.23.8 and
> 2.6.23.9
> 
> **************************************************************************
> *** Output from "vmstat -n 1 10" -- Note very high context switch rate ***
> *** This is on a idle machine!                                         ***
> **************************************************************************
> 
> procs -----------memory---------- ---swap-- -----io---- --system--
> ----cpu----
>  r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy
> id wa
>  0  0      0 1925556   4768 116104    0    0   124     2    6  7538  1  2
> 96  1
>  0  0      0 1925556   4768 116104    0    0     0     0    2 147329  0  1
> 99  0
>  0  0      0 1925548   4768 116104    0    0     0     0    0 154515  0  1
> 99  0
>  0  0      0 1925548   4768 116104    0    0     0     0    1 153898  0  2
> 98  0
>  0  0      0 1925548   4780 116104    0    0     0    16    3 155216  0  1
> 99  0
>  0  0      0 1925548   4780 116104    0    0     0     0    1 161718  0  1
> 99  0
>  0  0      0 1925548   4780 116104    0    0     0     0    0 147587  0  2
> 98  0
>  0  0      0 1925548   4780 116104    0    0     0     0    1 153524  0  2
> 98  0
>  0  0      0 1925448   4780 116104    0    0     0     0    0 153434  0  1
> 99  0
>  0  0      0 1925448   4792 116092    0    0     0    16    4 153527  0  2
> 98  0

So what piece of code is scheduling so much?  What does `top' say?  What
does the (sorted) output of oprofile look like?

Did you try shutting down as much userspace code as possible to find out if
some userspace task is misbehaving?

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