Re: CPU scheduler weirdness?

Marton Balint
2009-09-03T21:57:31Z

On Sat, 29 Aug 2009, Marton Balint wrote:

>
>
> On Thu, 20 Aug 2009, Marton Balint wrote:
>>
>>
>> On Thu, 20 Aug 2009, Ingo Molnar wrote:
>>
>>>
>>> * Marton Balint <cus@fazekas.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?

Ingo, Peter, could any of you guys have a look at the commit that caused
this bug? Is it OK to revert it? Or a fix somewhere else is necessary? I'm
pushing this because I hope that this bug will get fixed in the upcoming
stable kernel...

Regards,
Marton