From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vivek Goyal Subject: Re: [PATCH 07/11] blkcg: make request_queue bypassing on allocation Date: Fri, 13 Apr 2012 16:32:05 -0400 Message-ID: <20120413203205.GI26383@redhat.com> References: <1334347895-6268-1-git-send-email-tj@kernel.org> <1334347895-6268-8-git-send-email-tj@kernel.org> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <1334347895-6268-8-git-send-email-tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Tejun Heo Cc: axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org, ctalbott-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, rni-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@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