linux-kernel - Re: CPU scheduler weirdness?

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Date:	Wed, 19 Aug 2009 20:42:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Marton Balint <cus@...ekas.hu>
Cc:	Andreas Mohr <andi@...as.de>, linux-kernel@...r.kernel.org,
	mingo@...e.hu
Subject: Re: CPU scheduler weirdness?

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?

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