Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751571AbdIMCXJ (ORCPT ); Tue, 12 Sep 2017 22:23:09 -0400 Received: from mail-pg0-f42.google.com ([74.125.83.42]:36249 "EHLO mail-pg0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751423AbdIMCXH (ORCPT ); Tue, 12 Sep 2017 22:23:07 -0400 X-Google-Smtp-Source: ADKCNb5XMtfts3QzEBFC1wEeVT+qTBaDxUzuRVyYmaUUWWQ6yU13S9Bdt6vSdms5yu37/ZF8XRu9uA== Subject: Re: [PATCH] blk-mq: put the driver tag of nxt rq before first one is requeued To: "jianchao.wang" , Ming Lei Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <1505236475-9209-1-git-send-email-jianchao.w.wang@oracle.com> <20170912102302.GC15792@ming.t460p> <23b700aa-ed37-1081-d980-4073804f293b@oracle.com> <20170913012452.GA24642@ming.t460p> <4b7dcb10-9ec5-9a00-8d16-009e83ef7b5b@oracle.com> From: Jens Axboe Message-ID: Date: Tue, 12 Sep 2017 20:23:02 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <4b7dcb10-9ec5-9a00-8d16-009e83ef7b5b@oracle.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1889 Lines: 57 On 09/12/2017 07:39 PM, jianchao.wang wrote: > > > On 09/13/2017 09:24 AM, Ming Lei wrote: >> On Wed, Sep 13, 2017 at 09:01:25AM +0800, jianchao.wang wrote: >>> Hi ming >>> >>> On 09/12/2017 06:23 PM, Ming Lei wrote: >>>>> @@ -1029,14 +1029,20 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list) >>>>> if (list_empty(list)) >>>>> bd.last = true; >>>>> else { >>>>> - struct request *nxt; >>>>> - >>>>> nxt = list_first_entry(list, struct request, queuelist); >>>>> bd.last = !blk_mq_get_driver_tag(nxt, NULL, false); >>>>> } >>>>> >>>>> ret = q->mq_ops->queue_rq(hctx, &bd); >>>>> if (ret == BLK_STS_RESOURCE) { >>>>> + /* >>>>> + * If an I/O scheduler has been configured and we got a >>>>> + * driver tag for the next request already, free it again. >>>>> + */ >>>>> + if (!list_empty(list)) { >>>>> + nxt = list_first_entry(list, struct request, queuelist); >>>>> + blk_mq_put_driver_tag(nxt); >>>>> + } >>>> The following way might be more simple and clean: >>>> >>>> if (nxt) >>>> blk_mq_put_driver_tag(nxt); >>>> >>>> meantime 'nxt' need to be cleared inside the 'if (list_empty(list))' >>>> before .queue_rq(). >>> >>> I had ever thought about that, but to avoid add extra command in the >>> fast path, I made the patch above. >> >> Got it, so how about changing to the following way simply: >> >> if (nxt && !list_empty(list)) >> blk_mq_put_driver_tag(nxt); >> > It seems that we even could change it as following: > if (!list_empty(list)) > blk_mq_put_driver_tag(nxt); This is starting to get too clever for its own good, I generally don't like to sacrifice readability for performance. In reality, the compiler probably figures it out anyway... So either make it explicit, or add a nice comment as to why it is the way that it is. -- Jens Axboe