From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756766Ab2DMUcP (ORCPT ); Fri, 13 Apr 2012 16:32:15 -0400 Received: from mx1.redhat.com ([209.132.183.28]:7775 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753013Ab2DMUcO (ORCPT ); Fri, 13 Apr 2012 16:32:14 -0400 Date: Fri, 13 Apr 2012 16:32:05 -0400 From: Vivek Goyal To: Tejun Heo Cc: axboe@kernel.dk, ctalbott@google.com, rni@google.com, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, containers@lists.linux-foundation.org Subject: Re: [PATCH 07/11] blkcg: make request_queue bypassing on allocation 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 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1334347895-6268-8-git-send-email-tj@kernel.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.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