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

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Message-ID: <20120209155246.GD22552@somewhere.redhat.com>
Date:	Thu, 9 Feb 2012 16:52:49 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christoph Lameter <cl@...ux.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, Feb 02, 2012 at 10:29:57AM -0600, Christoph Lameter wrote:
> 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.

So this is a typical example of a timer we want to shutdown when the CPU is idle
but we want to keep it running when we run in adaptive tickless mode (ie: shutdown
the tick while the CPU is busy).
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