linux-kernel - Re: [PATCHSET] block, mempool, percpu: implement percpu mempool and fix blkcg percpu alloc deadlock

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Message-Id: <20111227142156.7943446e.akpm@linux-foundation.org>
Date:	Tue, 27 Dec 2011 14:21:56 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Vivek Goyal <vgoyal@...hat.com>, avi@...hat.com, nate@...nel.net,
	cl@...ux-foundation.org, oleg@...hat.com, axboe@...nel.dk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET] block, mempool, percpu: implement percpu mempool and
 fix blkcg percpu alloc deadlock

On Tue, 27 Dec 2011 14:07:53 -0800
Tejun Heo <tj@...nel.org> wrote:

> Hello,
> 
> On Tue, Dec 27, 2011 at 01:25:01PM -0800, Andrew Morton wrote:
> > umm, we've already declared that it is OK to completely waste this
> > memory for the users (probably a majority) who will not be using
> > these stats.
> 
> We're talking about combinatorial combinations where only small subset
> is usually expected to be used and, in addition to the absolute usage,
> there's big advantage in showing behavior which users would expect.
> If 1000 cgroups are doing IOs to 1000 devices, it's expected to
> consume some amount of resource.

<autorepeat>For those users who don't want the stats, stats shouldn't
consume any resources at all.

And I bet that the majority of the minority who want stats simply want
to know "how much IO is this cgroup doing", and don't need per-cgroup,
per-device accounting.

And it could be that the minority of the minority who want per-device,
per-cgroup stats only want those for a minority of the time.

IOW, what happens if we give 'em atomic_add() and be done with it?

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