linux-kernel - Re: CPU scheduler weirdness?

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Message-ID: <alpine.LNX.2.00.0908191353100.19120@cinke.fazekas.hu>
Date:	Wed, 19 Aug 2009 14:01:17 +0200 (CEST)
From:	Marton Balint <cus@...ekas.hu>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Andreas Mohr <andi@...as.de>, linux-kernel@...r.kernel.org,
	mingo@...e.hu
Subject: Re: CPU scheduler weirdness?

On Wed, 19 Aug 2009, Peter Zijlstra wrote:
> On Tue, 2009-08-18 at 21:49 +0200, Marton Balint wrote:
>
>> In the meantime, I was able to create a tiny C program which always
>> succesfully reproduces the bug. It's basically an endless loop which does
>> not stop while the process is running on the last CPU core. The program
>> creates multiple instances of itself, to be able to keep all of the CPU
>> cores busy. After 1 second, the processes running on other than the last
>> CPU core die, the processes running on the last CPU core remain stuck
>> there...
>>
>> I tested it on my dual core system, if someone could test it on a quad
>> core and report back that would probably be useful.
>>
>> Usage: ./schedtest <number of CPU cores>
>>
>> And don't forget to kill the stuck processes after using the program! :)
>
> So what's the bug? Sure one task will stay on the cpu, and because there
> is no contention it doesn't get migrated, and therefore won't quit,
> how's that a problem?

Problem is that more than one processes remain on that CPU core, and none 
of them get migrated to other (idle) cores. I tested it with my E8400 
processor and 2.6.31-rc5-git3 kernel.

Regards,
   Marton
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