Date: Tue, 8 May 2012 10:24:29 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
To: Gilad Ben-Yossef <gilad@benyossef.com>
cc: linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Tejun Heo <tj@kernel.org>, John Stultz <johnstul@us.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Mel Gorman <mel@csn.ul.ie>, Mike Frysinger <vapier@gentoo.org>,
        David Rientjes <rientjes@google.com>, Hugh Dickins <hughd@google.com>,
        Minchan Kim <minchan.kim@gmail.com>,
        Konstantin Khlebnikov <khlebnikov@openvz.org>,
        Chris Metcalf <cmetcalf@tilera.com>,
        Hakan Akkan <hakanakkan@gmail.com>,
        Max Krasnyansky <maxk@qualcomm.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, linux-mm@kvack.org
Subject: Re: [PATCH v1 5/6] mm: make vmstat_update periodic run conditional
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On Tue, 8 May 2012, Gilad Ben-Yossef wrote:

> My line of thought was that if we explicitly choose a scapegoat cpu we
> and the user need to manage this - such as worry about what happens if
> the scapegoats is offlines and let the user explicitly designate the
> scapegoat cpu thus creating another knob, and worrying about what
> happens if the user designate such a cpu but then it goes offlines...

The scapegoat can be chosen on boot. One can f.e. create a file in

/sys/device/syste/cpu called "scapegoat" which contains the number of the
processor chosen. Then one can even write a userspace daemon to automatize
the moving of the processing elsewhere. Could be integrated into something
horrible like irqbalance f.e.

> I figured the user needs to worry about other unbounded work items
> anyway if he cares about where such things are run in the general case,
> but using isolcpus for example.

True. So the scapegoat heuristic could be to pick the first
unisolated cpu.

> The same should be doable with cpusets, except that right now we mark
> unbounded workqueue worker threads as pinned even though they aren't. If
> I understood the discussion, the idea is exactly to stop users from
> putting these threads in non root cpusets. I am not 100% sure why..

Not sure that cpusets is a good thing to bring in here because that is an
optional feature of the kernel and tying basic functionality like this
to cpuset support does not sound right to me.
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