From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756763Ab2DMUhc (ORCPT ); Fri, 13 Apr 2012 16:37:32 -0400 Received: from mail-pz0-f52.google.com ([209.85.210.52]:35676 "EHLO mail-pz0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753013Ab2DMUha (ORCPT ); Fri, 13 Apr 2012 16:37:30 -0400 Date: Fri, 13 Apr 2012 13:37:26 -0700 From: Tejun Heo To: Vivek Goyal 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: <20120413203726.GE12233@google.com> References: <1334347895-6268-1-git-send-email-tj@kernel.org> <1334347895-6268-8-git-send-email-tj@kernel.org> <20120413203205.GI26383@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120413203205.GI26383@redhat.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 13, 2012 at 04:32:05PM -0400, Vivek Goyal wrote: > 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. blk_cleanup_queue() doesn't use blk_queue_bypass_start() to enter bypass mode. Thanks. -- tejun