linux-kernel - Re: High priority tasks break SMP balancer?

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Date:	Fri, 9 Nov 2007 16:11:03 -0800
From:	Micah Dowty <micah@...are.com>
To:	Cyrus Massoumi <cyrusm@....net>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: High priority tasks break SMP balancer?

On Sat, Nov 10, 2007 at 12:56:07AM +0100, Cyrus Massoumi wrote:
> I tried your program on my machine (C2D, 2.6.17, O(1) scheduler).
>
> Both CPUs are 100% busy all the time. Each busy-looping thread is running 
> on its own CPU. I've been watching top output for 10 minutes, the spreading 
> is stable and the threads don't bounce at all.

As I said, YMMV. I haven't been able to find a single set of
parameters for the demo program which cause the problem to occur 100%
of the time on all systems.

In general, boosting the MAINTHREAD_PRIORITY even more and increasing
the WAKE_HZ should exaggerate the problem. These parameters reproduce
the problem very reliably on my system:

#define NUM_BUSY_THREADS            2
#define MAINTHREAD_PRIORITY       -20
#define MAINTHREAD_WAKE_HZ       1024
#define MAINTHREAD_LOAD_PERCENT     5
#define MAINTHREAD_LOAD_CYCLES      2

It's also possible this problem doesn't occur on 2.6.17. I have only
tested this example on 2.6.23.1 and 2.6.20 so far.

Thanks,
--Micah
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