From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752238AbbEDTrF (ORCPT ); Mon, 4 May 2015 15:47:05 -0400 Received: from mx0a-00082601.pphosted.com ([67.231.145.42]:27014 "EHLO mx0a-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750899AbbEDTq6 (ORCPT ); Mon, 4 May 2015 15:46:58 -0400 Message-ID: <5547CCA9.2000003@fb.com> Date: Mon, 4 May 2015 13:46:49 -0600 From: Jens Axboe User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 MIME-Version: 1.0 To: Shaohua Li , Jeff Moyer CC: , , Subject: Re: [PATCH 4/5] blk-mq: do limited block plug for multiple queue case References: <20150504194037.GA3300441@devbig257.prn2.facebook.com> In-Reply-To: <20150504194037.GA3300441@devbig257.prn2.facebook.com> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [192.168.54.13] X-Proofpoint-Spam-Reason: safe X-FB-Internal: Safe X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:5.13.68,1.0.33,0.0.0000 definitions=2015-05-04_04:2015-05-04,2015-05-04,1970-01-01 signatures=0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/04/2015 01:40 PM, Shaohua Li wrote: > On Fri, May 01, 2015 at 04:16:04PM -0400, Jeff Moyer wrote: >> Shaohua Li writes: >> >>> plug is still helpful for workload with IO merge, but it can be harmful >>> otherwise especially with multiple hardware queues, as there is >>> (supposed) no lock contention in this case and plug can introduce >>> latency. For multiple queues, we do limited plug, eg plug only if there >>> is request merge. If a request doesn't have merge with following >>> request, the requet will be dispatched immediately. >>> >>> This also fixes a bug. If we directly issue a request and it fails, we >>> use blk_mq_merge_queue_io(). But we already assigned bio to a request in >>> blk_mq_bio_to_request. blk_mq_merge_queue_io shouldn't run >>> blk_mq_bio_to_request again. >> >> Good catch. Might've been better to split that out first for easy >> backport to stable kernels, but I won't hold you to that. > > It's not a severe bug, but I don't mind. Jens, please let me know if I > should split the patch into 2 patches. I don't care that much for this particular case. But since one/more of the others need respin anyway, might be prudent to split it up in any case. -- Jens Axboe