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

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Message-ID: <20071116060700.GD16273@elte.hu>
Date:	Fri, 16 Nov 2007 07:07:00 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Micah Dowty <micah@...are.com>
Cc:	Christoph Lameter <clameter@....com>,
	Kyle Moffett <mrmacman_g4@....com>,
	Cyrus Massoumi <cyrusm@....net>,
	LKML Kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, Mike Galbraith <efault@....de>,
	Paul Menage <menage@...gle.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Peter Williams <pwil3058@...pond.net.au>
Subject: Re: High priority tasks break SMP balancer?


* Micah Dowty <micah@...are.com> wrote:

> > I am a bit at a loss as to how this could relate to the patch. This 
> > looks like a load balance logic issue that causes the load 
> > calculation to go wrong?
> 
> My best guess is that this has something to do with the timing with 
> which we sample the CPU's instantaneous load when calculating the load 
> averages.. but I still understand only the basics of the scheduler and 
> SMP balancer. All I really know for sure at this point regarding your 
> patch is that git-bisect found it for me.

hm, your code uses timeouts for this, right? The CPU load average that 
is used for SMP load balancing is sampled from the scheduler tick - and 
has been sampled from the scheduler tick for eons. v2.6.23 defaulted to 
a different method but v2.6.24 samples it from the tick again. So my 
guess is, your testcode behave similarly on 2.6.22 too, correct?

	Ingo
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