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

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Date:	Tue, 24 Jun 2014 17:43:40 -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.

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.

Thanks.

-- 
tejun
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