linux-kernel - Re: CPU scheduler weirdness?

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Message-ID: <20090820105645.GA23635@elte.hu>
Date:	Thu, 20 Aug 2009 12:56:45 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Marton Balint <cus@...ekas.hu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andreas Mohr <andi@...as.de>, linux-kernel@...r.kernel.org
Subject: Re: CPU scheduler weirdness?


* Marton Balint <cus@...ekas.hu> wrote:

>
> On Wed, 19 Aug 2009, Peter Zijlstra wrote:
>
>> On Wed, 2009-08-19 at 14:34 +0200, Marton Balint wrote:
>>>
>>> On Wed, 19 Aug 2009, Peter Zijlstra wrote:
>>>
>>>> On Wed, 2009-08-19 at 14:01 +0200, Marton Balint wrote:
>>>>> 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.
>>>>
>>>> Only one remains here.. on a c2q running 2.6.31-rc6-tip
>>>>
>>>> Do you have a .config handy?
>>>>
>>>
>>> Yes it's in my original post:
>>>
>>> http://marc.info/?l=linux-kernel&m=125012584709800&w=2
>>
>> Right you are,.. so I build a kernel with the cgroup scheduler in and
>> tested it on a dual-core opteron machine, but I can't seem to reproduce
>> this.
>>
>> Are you using cgroups in any way, or do you simply have it enabled in
>> your config?
>
> No, it's just enabled. Actually the kernel is from the 
> openSUSE build service:
>
> http://download.opensuse.org/repositories/Kernel:/HEAD/openSUSE_11.1/x86_64/
>
> But the problem is present for both the kernel-default 
> kernel and the kernel-vanilla kernel which does not 
> contain any suse-specific patches.
>
> This evening I had a bit more time to test, and I've 
> made a surprising discovery: I can only reproduce the 
> bug if the kernel module of my TV tuner card is loaded. 
> I have a Leadtek Winfast 2000 XP Expert TV card, it 
> uses the cx8800 kernel module. It seems that the 
> problem is somehow related to the infrared sensor of 
> the TV card, because I recompiled the module with the 
> 'case CX88_BOARD_WINFAST2000XP_EXPERT:' line removed 
> from cx88-input.c and I couldn't reproduce the bug with 
> the new kernel module.

Extremely weird. Are timers somehow busted?

	Ingo
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