Re: CPU scheduler weirdness?

[Marton Balint]

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.

Regards,
  Marton
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