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Date:	Sun, 1 Jan 2012 10:03:42 +0200
From:	Gilad Ben-Yossef <gilad@...yossef.com>
To:	Mel Gorman <mgorman@...e.de>, Chris Metcalf <cmetcalf@...era.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Russell King <linux@....linux.org.uk>, 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>
Subject: Re: [PATCH v4 5/5] mm: Only IPI CPUs to drain local pages if they exist

On Fri, Dec 30, 2011 at 10:29 PM, Gilad Ben-Yossef <gilad@...yossef.com> wrote:
> On Fri, Dec 30, 2011 at 6:08 PM, Mel Gorman <mgorman@...e.de> wrote:
>> On Fri, Dec 30, 2011 at 10:25:46AM -0500, Chris Metcalf wrote:
>
>>> Alternately, since we really don't want more than one cpu running the drain
>>> code anyway, you could imagine using a static cpumask, along with a lock to
>>> serialize attempts to drain all the pages.  (Locking here would be tricky,
>>> since we need to run on_each_cpu with interrupts enabled, but there's
>>> probably some reasonable way to make it work.)
>>>
>>
>> Good suggestion, that would at least shut up my complaining
>> about allocation costs! A statically-declared mutex similar
>> to hugetlb_instantiation_mutex should do it. The context that
>> drain_all_pages is called from will have interrupts enabled.
>>
>> Serialising processes entering direct reclaim may result in some stalls
>> but overall I think the impact of that would be less than increasing
>> memory pressure when low on memory.
>>
>
> Chris, I like the idea :-)
>
> Actually, assuming for a second that on_each_cpu* and underlying code
> wont mind if the cpumask will change mid call (I know it does, just thinking out
> loud), you could say you don't even need the lock if you careful in how you
> set/unset the per cpu bits of the cpumask, since they track the same thing...

I took a look and smp_call_function_many is actually fine with the
passed cpumask getting changed in mid call.

I think this means we can do away with a single global cpumask without
any locking and the cost becomes the allocation space for the single cpumask and
the cache bouncing for concurrent updating of the cpumask if
drain_all_pages races
 against itself on other cpus.

I'll spin a patch based on this idea.

Happy new year :-)
Gilad


-- 
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@...yossef.com
Israel Cell: +972-52-8260388
US Cell: +1-973-8260388
http://benyossef.com

"Unfortunately, cache misses are an equal opportunity pain provider."
-- Mike Galbraith, LKML
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