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

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Message-ID: <20071116091904.GB31219@vmware.com>
Date:	Fri, 16 Nov 2007 01:19:04 -0800
From:	Micah Dowty <micah@...are.com>
To:	Ingo Molnar <mingo@...e.hu>
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?

On Fri, Nov 16, 2007 at 07:07:00AM +0100, Ingo Molnar wrote:
> > 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?

My sample program (and the VMware code which I originally observed
this behaviour with) wakes up frequently using /dev/rtc.

I know the problem occurs on 2.6.20 and 2.6.23.1. I haven't tried
2.6.22 yet, but I'll do so as soon as I can.

Thanks,
--Micah
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