linux-kernel - Re: [PATCH 07/11] blkcg: make request

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Message-ID: <20120413203205.GI26383@redhat.com>
Date:	Fri, 13 Apr 2012 16:32:05 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	axboe@...nel.dk, ctalbott@...gle.com, rni@...gle.com,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	containers@...ts.linux-foundation.org
Subject: Re: [PATCH 07/11] blkcg: make request_queue bypassing on allocation

On Fri, Apr 13, 2012 at 01:11:31PM -0700, Tejun Heo wrote:
> With the previous change to guarantee bypass visiblity for RCU read
> lock regions, entering bypass mode involves non-trivial overhead and
> future changes are scheduled to make use of bypass mode during init
> path.  Combined it may end up adding noticeable delay during boot.
> 
> This patch makes request_queue start its life in bypass mode, which is
> ended on queue init completion at the end of
> blk_init_allocated_queue(), and updates blk_queue_bypass_start() such
> that draining and RCU synchronization are performed only when the
> queue actually enters bypass mode.
> 
> This avoids unnecessarily switching in and out of bypass mode during
> init avoiding the overhead and any nasty surprises which may step from
> leaving bypass mode on half-initialized queues.

Tejun, I am not sure that this will fix the problem completely. I think
we will still face the overhead of synchronize_rcu() in
blkcg_deactivate_policy() as it will be called from cfq_exit_queue() for
initialized queues.

In the past I had I had used synchronize_rcu() in cfq_exit_queue() and
noticed the overhead. Looks like driver was creating fully initialized
queues and tearing these apart soon.

Thanks
Vivek
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