linux-kernel - Re: [PATCHSET] mempool, percpu, blkcg: fix percpu stat allocation and remove stats

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Date:	Thu, 8 Mar 2012 12:35:35 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, axboe@...nel.dk,
	hughd@...gle.com, avi@...hat.com, nate@...nel.net,
	cl@...ux-foundation.org, linux-kernel@...r.kernel.org,
	dpshah@...gle.com, ctalbott@...gle.com, rni@...gle.com
Subject: Re: [PATCHSET] mempool, percpu, blkcg: fix percpu stat allocation
 and remove stats_lock

Hello, Vivek.

On Thu, Mar 08, 2012 at 03:33:31PM -0500, Vivek Goyal wrote:
> Ok, I can confirm that it is cfqq reference on blkg which is an issue. If
> I move my shell to a child cgroup and try to do some operations (in the
> context of shell, like autocompletion/reading an uncached dir), then IO
> is issued in the context of shell, I move out the shell out of cgroup and
> then try to delete it, it hangs. Once I exit out of shell, blkg reference
> is dropped and cgroup is deleted.
> 
> So we do need to cleanup the cic->cfqq upon cgroup change synchronously.

I see.  Thanks for confirming the problem.

> That will still not solve the issue of a process dumping tons of
> IO on device (large nr_requests) and then moving out of cgroup. Now
> cgroup deletion will still hang till all the IO in the cgroup
> completes.

I don't think the latter is too much of a problem.  I've been meaning
to decouple destruction from ref putting anyway, so let's see whether
that's possible.

Thanks.

-- 
tejun
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