linux-kernel - Re: [v7 0/8] Reduce cross CPU IPI interference

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Date:	Thu, 2 Feb 2012 10:29:57 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Gilad Ben-Yossef <gilad@...yossef.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, Chris Metcalf <cmetcalf@...era.com>,
	linux-mm@...ck.org, Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Avi Kivity <avi@...hat.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Kosaki Motohiro <kosaki.motohiro@...il.com>,
	Milton Miller <miltonm@....com>
Subject: Re: [v7 0/8] Reduce cross CPU IPI interference

On Thu, 2 Feb 2012, Frederic Weisbecker wrote:

> > Some pinned timers might be able to get special treatment as well - take for
> > example the vmstat work being schedule every second, what should we do with
> > it for CPU isolation?
>
> Right, I remember I saw these vmstat timers on my way when I tried to get 0
> interrupts on a CPU.
>
> I think all these timers need to be carefully reviewed before doing anything.
> But we certainly shouldn't adopt the behaviour of migrating timers by default.
>
> Some timers really needs to stay on the expected CPU. Note that some
> timers may be shutdown by CPU hotplug callbacks. Those wouldn't be migrated
> in case of CPU offlining. We need to keep them.
>
> > It makes sense to me to have that stop scheduling itself when we have the tick
> > disabled for both idle and a nohz task.

The vmstat timer only makes sense when the OS is doing something on the
processor. Otherwise if no counters are incremented and the page and slab
allocator caches are empty then there is no need to run the vmstat timer.