linux-kernel - Re: [rfc] lru_add_drain

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Message-ID: <alpine.DEB.1.10.0909081110450.30203@V090114053VZO-1>
Date:	Tue, 8 Sep 2009 11:22:08 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...e.hu>,
	linux-mm <linux-mm@...ck.org>,
	Oleg Nesterov <onestero@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] lru_add_drain_all() vs isolation

On Tue, 8 Sep 2009, Peter Zijlstra wrote:

> There is _no_ functional difference between before and after, except
> less wakeups on cpus that don't have any __lru_cache_add activity.
>
> If there's pages on the per cpu lru_add_pvecs list it will be present in
> the mask and will be send a drain request. If its not, then it won't be
> send.

Ok I see.

A global cpu mask like this will cause cacheline bouncing. After all this
is a hot cpu path. Maybe do not set the bit if its already set
(which may be very frequent)? Then add some benchmarks to show that it
does not cause a regression on a 16p box (Nehalem) or so?






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