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

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Message-ID: <20071116213840.GA31527@vmware.com>
Date:	Fri, 16 Nov 2007 13:38:40 -0800
From:	Micah Dowty <micah@...are.com>
To:	David Newall <david@...idnewall.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>,
	Christoph Lameter <clameter@....com>
Subject: Re: High priority tasks break SMP balancer?

On Sat, Nov 17, 2007 at 05:43:33AM +1030, David Newall wrote:
> There are a couple of points I would make about your python test harness.  
> Your program compares real+system jiffies for both cpus; an ideal result 
> would be 1.00.  The measurement is taken over a relatively short period of 
> approximately a half-second, and you kill the CPU hogs before taking final 
> measurements, even wait for them to die first.  You repeat this 
> measurement, starting and killing CPU hogs each time.  Why do you do that?

The Python test harness is fairly artificial, but this is just the
best way I found to reliably reproduce the problem in a short amount
of time. It was just for convenience while running git-bisect. When
running the C program directly, there seems to be a somewhat random
chance that it will start up in the "bad" state. Once the single CPU
is stuck in this mostly-idle mode, it seems to stay that way for a
while.

> What happens if you start the hogs and take the baseline outside of the 
> loop?

The problem still occurs then, but killing/restarting the test app
seems to trigger the problem more reliably. As I said in the original
email about this, left to its own devices this problem will occur
seemingly-randomly. In the original VMware code I observed this
problem in, the same process would flip between the "good" and "bad"
states seemingly randomly, every few seconds.

Thanks,
--Micah

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