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Date: Thu, 9 Feb 2012 10:26:02 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
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To: Frederic Weisbecker <fweisbec@gmail.com>
cc: Gilad Ben-Yossef <gilad@benyossef.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, linux-kernel@vger.kernel.org,
        Chris Metcalf <cmetcalf@tilera.com>, linux-mm@kvack.org,
        Pekka Enberg <penberg@kernel.org>, Matt Mackall <mpm@selenic.com>,
        Sasha Levin <levinsasha928@gmail.com>, Rik van Riel <riel@redhat.com>,
        Andi Kleen <andi@firstfloor.org>, Mel Gorman <mel@csn.ul.ie>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>, Avi Kivity <avi@redhat.com>,
        Michal Nazarewicz <mina86@mina86.com>,
        Kosaki Motohiro <kosaki.motohiro@gmail.com>,
        Milton Miller <miltonm@bga.com>
Subject: Re: [v7 0/8] Reduce cross CPU IPI interference
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On Thu, 9 Feb 2012, Frederic Weisbecker wrote:

> > 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).

You can also shut it down when the cpu is busy and not doing any system
calls. If the percpu differentials are all zero (because you just ran the
timer f.e.) and there are no system activities that would change the
counters then there is no point in running the vmstat timer.