linux-kernel - Re: CFS review

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Message-ID: <20070801112229.GA11710@elte.hu>
Date:	Wed, 1 Aug 2007 13:22:29 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Roman Zippel <zippel@...ux-m68k.org>
Cc:	Mike Galbraith <efault@....de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: CFS review

* Roman Zippel <zippel@...ux-m68k.org> wrote:

> [...] e.g. in this example there are three tasks that run only for 
> about 1ms every 3ms, but they get far more time than should have 
> gotten fairly:
> 
>  4544 roman     20   0  1796  520  432 S 32.1  0.4   0:21.08 lt
>  4545 roman     20   0  1796  344  256 R 32.1  0.3   0:21.07 lt
>  4546 roman     20   0  1796  344  256 R 31.7  0.3   0:21.07 lt
>  4547 roman     20   0  1532  272  216 R  3.3  0.2   0:01.94 l

Mike and me have managed to reproduce similarly looking 'top' output, 
but it takes some effort: we had to deliberately run a non-TSC 
sched_clock(), CONFIG_HZ=100, !CONFIG_NO_HZ and !CONFIG_HIGH_RES_TIMERS.

in that case 'top' accounting symptoms similar to the above are not due 
to the scheduler starvation you suspected, but due the effect of a 
low-resolution scheduler clock and a tightly coupled timer/scheduler 
tick to it. I tried the very same workload on 2.6.22 (with the same 
.config) and i saw similarly anomalous 'top' output. (Not only can one 
create really anomalous CPU usage, one can completely hide tasks from 
'top' output.)

if your test-box has a high-resolution sched_clock() [easily possible] 
then please send us the lt.c and l.c code so that we can have a look.

	Ingo
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