linux-kernel - Re: [PATCH] blkio: Release blkg infrastructure only after last policy is deactivated.

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Date:	Fri, 4 Jul 2014 13:18:24 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Shirish Pargaonkar <shirishpargaonkar@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, vgoyal@...hat.com,
	Shirish Pargaonkar <spargaonkar@...e.com>, axboe@...nel.dk
Subject: Re: [PATCH] blkio: Release blkg infrastructure only after last
 policy is deactivated.

On Tue, Jun 24, 2014 at 05:43:40PM -0400, Tejun Heo wrote:
> Hello,
> 
> On Tue, Jun 24, 2014 at 04:37:58PM -0500, Shirish Pargaonkar wrote:
> > When we start from blk_cleanup_queue(), we put request queue in bypass mode,
> > drain it (and service queues), and then destroy blkcgs (explicitly)
> > 
> > When we start from blk_release_queue(), we do not drain first and then
> > destroy blkcgs.  So if we destroy blkcg and then call (implicitly) and
> > bail out of
> > blk_drain_queue, we would not have drained the service queues which
> > is not what we want.
> 
> I'm not really following you.  What do you mean "when we start from
> blk_release_queue()"?  blk_release_queue() is called after the last
> put which can only follow blk_cleanup_queue() if the queue is fully
> initialized.  The queue is already in bypass mode and fully drained by
> the time control reaches blk_release_queue().  Module [un]load
> re-invoking the path doesn't change anything.
> 
> > I do not see any harm in waiting till end to release blkcgs (as I understand).
> 
> Well, the harm there is not freeing those blkgs unless all the blkcg
> policies are unloaded which is usually never on most systems.

Ping.  We have a patch which makes this problem more visible.  Are you
still planning to re-spin the patch?

Thanks.

-- 
tejun
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