Re: [PATCH 07/11] blkcg: make request_queue bypassing on allocation

[Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

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