linux-kernel - Re: CPU scheduler weirdness?

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Date:	Sat, 29 Aug 2009 16:15:14 +0200 (CEST)
From:	Marton Balint <cus@...ekas.hu>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Andreas Mohr <andi@...as.de>, linux-kernel@...r.kernel.org
Subject: Re: CPU scheduler weirdness?



On Thu, 20 Aug 2009, Marton Balint wrote:
>
>
> On Thu, 20 Aug 2009, Ingo Molnar wrote:
>
>> 
>> * 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?
>
> How can I check that?
>
> In the meantime, I updated my original C program and also created a kernel 
> module (schedtest_mod.c) which causes the same scheduling problems as the 
> kernel module of my TV card. The kernel module is a skeleton of the infrared 
> sensor polling code in cx88-input.c. It uses schedule_delayed_work, this 
> seems to cause the problem. The C program (schedtest.c) is also updated, it 
> now detects the number of CPU cores, from now, what you can set as a command 
> line parameter is the CPU core number, on which the schedtest processes will 
> not quit. (previously this was always the last core).
>
> So to reproduce the bug on a dual core system, compile and insert the kernel 
> module (schedtest_mod.c). Then check dmesg, it should contain on which CPU 
> core is the delayed_work running. You should use the CPU core id of the 
> _other_ CPU core as a command line parameter to the updated schedtest 
> program.
>
> And by the way, thank you guys for the help so far, hopefully we'll get to 
> the bottom of this :)

I reproduced the bug with the previously provided kernel module and 
C program on a different computer (it's a laptop with a core2 duo P8400 
CPU), and also bisected the bug to this commit:

sched: fine-tune SD_MC_INIT:
14800984706bf6936bbec5187f736e928be5c218

If I add again the removed SD_BALANCE_NEWIDLE to flags, then everything 
works as expected. So what would be the correct fix for this bug? 
Revert the patch? Or just add SD_BALANCE_NEWIDLE to flags?

Regards,
   Marton
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