linux-kernel - Re: CPU scheduler weirdness?

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Message-Id: <1250665455.7583.326.camel@twins>
Date:	Wed, 19 Aug 2009 09:04:15 +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 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?

If you start a bunch of loops (enough to fill all cpus) you'll find
it'll get migrated and die pretty quickly.

Those same loops `while :; do :; done &` get spread around the available
cores just fine, still no bug.

btw: sysconf(_SC_NPROCESSORS_ONLN);
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