From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753646AbaIXRd5 (ORCPT ); Wed, 24 Sep 2014 13:33:57 -0400 Received: from mail-vc0-f177.google.com ([209.85.220.177]:64906 "EHLO mail-vc0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752130AbaIXRd4 (ORCPT ); Wed, 24 Sep 2014 13:33:56 -0400 Date: Wed, 24 Sep 2014 13:33:52 -0400 From: Tejun Heo To: Christoph Hellwig Cc: Jens Axboe , linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org Subject: Re: [PATCH block/for-3.18/core] blk-mq: start q->mq_usage_counter in atomic mode Message-ID: <20140924173352.GC29067@mtj.dyndns.org> References: <20140919113815.GA10791@lst.de> <20140923060812.GH11740@mtj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140923060812.GH11740@mtj.dyndns.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 23, 2014 at 02:08:12AM -0400, Tejun Heo wrote: > From 83b06f4fc6ca2f7f3d706a168b71c248bdada668 Mon Sep 17 00:00:00 2001 > From: Tejun Heo > Date: Tue, 23 Sep 2014 01:58:34 -0400 > > blk-mq uses percpu_ref for its usage counter which tracks the number > of in-flight commands and used to synchronously drain the queue on > freeze. percpu_ref shutdown takes measureable wallclock time as it > involves a sched RCU grace period. This means that draining a blk-mq > takes measureable wallclock time. One would think that this shouldn't > matter as queue shutdown should be a rare event which takes place > asynchronously w.r.t. userland. Applied to percpu/for-3.18 on top of the klude, its revert and the required percpu_ref changes. Thanks. -- tejun