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

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Message-ID: <20071115202425.GC4914@vmware.com>
Date:	Thu, 15 Nov 2007 12:24:25 -0800
From:	Micah Dowty <micah@...are.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Kyle Moffett <mrmacman_g4@....com>,
	Cyrus Massoumi <cyrusm@....net>,
	LKML Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Andrew Morton <akpm@...l.org>,
	Mike Galbraith <efault@....de>, Paul Menage <menage@...gle.com>
Subject: Re: High priority tasks break SMP balancer?

On Thu, Nov 15, 2007 at 12:07:47PM -0800, Christoph Lameter wrote:
> On Thu, 15 Nov 2007, Micah Dowty wrote:
> 
> > For reference, the exact test I used with git-bisect is attached. The
> > C program (priosched) starts two busy-looping threads and a
> > high-priority high-frequency thread which uses relatively little
> > CPU. The Python program repeatedly starts the C program, runs it for a
> > half second, and measures the resulting imbalance in CPU usage. On
> > kernels prior to the above commit, this reports values within about
> > 10% of 1.0. On later kernels, it crashes within a couple iterations
> > due to a divide-by-zero error :)
> 
> The kernel crashes? Sounds like your application crashes with a divide by 
> zero?

Yes, the Python test harness crashes, not the kernel. It's just
because on a kernel which exhibits this SMP balancer bug, within a
couple of test iterations I'll hit a case where cpu1 was almost
totally idle and the test harness divides by zero when calculating the
imbalance.

--Micah
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