Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751798AbdIMCmu (ORCPT ); Tue, 12 Sep 2017 22:42:50 -0400 Received: from aserp1040.oracle.com ([141.146.126.69]:41601 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751732AbdIMCms (ORCPT ); Tue, 12 Sep 2017 22:42:48 -0400 Subject: Re: [PATCH] blk-mq: put the driver tag of nxt rq before first one is requeued To: Jens Axboe , 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: "jianchao.wang" Message-ID: <96a21c69-cf79-58a2-2c71-2e440abf9b36@oracle.com> Date: Wed, 13 Sep 2017 10:42:31 +0800 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: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Source-IP: aserv0021.oracle.com [141.146.126.233] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2288 Lines: 68 On 09/13/2017 10:23 AM, Jens Axboe wrote: > 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. > yes, it indeed leads to compiler warning of "may be used uninitialized" maybe the original one could be taken back. if (!list_empty(list)) { nxt = list_first_entry(list, struct request, queuelist); blk_mq_put_driver_tag(nxt); } It is more readable and could avoid the warning. Thanks jianchao