From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 30A0AC433EF for ; Tue, 26 Apr 2022 11:27:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1349539AbiDZLan (ORCPT ); Tue, 26 Apr 2022 07:30:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56144 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1349536AbiDZLag (ORCPT ); Tue, 26 Apr 2022 07:30:36 -0400 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C8F0F6F4BD; Tue, 26 Apr 2022 04:27:27 -0700 (PDT) Received: from kwepemi100003.china.huawei.com (unknown [172.30.72.54]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4Knfj46hcqzfbFj; Tue, 26 Apr 2022 19:26:32 +0800 (CST) Received: from kwepemm600009.china.huawei.com (7.193.23.164) by kwepemi100003.china.huawei.com (7.221.188.122) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Tue, 26 Apr 2022 19:27:25 +0800 Received: from [10.174.176.73] (10.174.176.73) by kwepemm600009.china.huawei.com (7.193.23.164) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Tue, 26 Apr 2022 19:27:25 +0800 Subject: Re: [PATCH -next v2 2/5] block, bfq: add fake weight_counter for weight-raised queue To: Jan Kara CC: , , , , , , References: <20220416093753.3054696-1-yukuai3@huawei.com> <20220416093753.3054696-3-yukuai3@huawei.com> <20220425094856.qgkhba2klguduxot@quack3.lan> <20220425161650.xzyijgkb5yzviea3@quack3.lan> <4591d02d-1f14-c928-1c50-6e434dfbb7b2@huawei.com> <20220426074023.5y4gwvjsjzem3vgp@quack3.lan> <77b4c06c-f813-bcac-ea26-107e52f46d0a@huawei.com> <20220426091556.qzryd552gzo6dikf@quack3.lan> From: "yukuai (C)" Message-ID: <6a829b09-4546-990a-52b6-bbd398f864bb@huawei.com> Date: Tue, 26 Apr 2022 19:27:24 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20220426091556.qzryd552gzo6dikf@quack3.lan> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.176.73] X-ClientProxiedBy: dggems702-chm.china.huawei.com (10.3.19.179) To kwepemm600009.china.huawei.com (7.193.23.164) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 在 2022/04/26 17:15, Jan Kara 写道: > On Tue 26-04-22 16:27:46, yukuai (C) wrote: >> 在 2022/04/26 15:40, Jan Kara 写道: >>> On Tue 26-04-22 09:49:04, yukuai (C) wrote: >>>> 在 2022/04/26 0:16, Jan Kara 写道: >>>>> Hello! >>>>> >>>>> On Mon 25-04-22 21:34:16, yukuai (C) wrote: >>>>>> 在 2022/04/25 17:48, Jan Kara 写道: >>>>>>> On Sat 16-04-22 17:37:50, Yu Kuai wrote: >>>>>>>> Weight-raised queue is not inserted to weights_tree, which makes it >>>>>>>> impossible to track how many queues have pending requests through >>>>>>>> weights_tree insertion and removel. This patch add fake weight_counter >>>>>>>> for weight-raised queue to do that. >>>>>>>> >>>>>>>> Signed-off-by: Yu Kuai >>>>>>> >>>>>>> This is a bit hacky. I was looking into a better place where to hook to >>>>>>> count entities in a bfq_group with requests and I think bfq_add_bfqq_busy() >>>>>>> and bfq_del_bfqq_busy() are ideal for this. It also makes better sense >>>>>>> conceptually than hooking into weights tree handling. >>>>>> >>>>>> bfq_del_bfqq_busy() will be called when all the reqs in the bfqq are >>>>>> dispatched, however there might still some reqs are't completed yet. >>>>>> >>>>>> Here what we want to track is how many bfqqs have pending reqs, >>>>>> specifically if the bfqq have reqs are't complted. >>>>>> >>>>>> Thus I think bfq_del_bfqq_busy() is not the right place to do that. >>>>> >>>>> Yes, I'm aware there will be a difference. But note that bfqq can stay busy >>>>> with only dispatched requests because the logic in __bfq_bfqq_expire() will >>>>> not call bfq_del_bfqq_busy() if idling is needed for service guarantees. So >>>>> I think using bfq_add/del_bfqq_busy() would work OK. >>>> Hi, >>>> >>>> I didn't think of that before. If bfqq stay busy after dispathing all >>>> the requests, there are two other places that bfqq can clear busy: >>>> >>>> 1) bfq_remove_request(), bfqq has to insert a new req while it's not in >>>> service. >>> >>> Yes and the request then would have to be dispatched or merged. Which >>> generally means another bfqq from the same bfqg is currently active and >>> thus this should have no impact on service guarantees we are interested in. >>> >>>> 2) bfq_release_process_ref(), user thread is gone / moved, or old bfqq >>>> is gone due to merge / ioprio change. >>> >>> Yes, here there's no new IO for the bfqq so no point in maintaining any >>> service guarantees to it. >>> >>>> I wonder, will bfq_del_bfqq_busy() be called immediately when requests >>>> are completed? (It seems not to me...). For example, a user thread >>>> issue a sync io just once, and it keep running without issuing new io, >>>> then when does the bfqq clears the busy state? >>> >>> No, when bfqq is kept busy, it will get scheduled as in-service queue in >>> the future. Then what happens depends on whether it will get more requests >>> or not. But generally its busy state will get cleared once it is expired >>> for other reason than preemption. >> >> Thanks for your explanation. >> >> I think in normal case using bfq_add/del_bfqq_busy() if fine. >> >> There is one last situation that I'm worried: If some disk are very >> slow that the dispatched reqs are not completed when the bfqq is >> rescheduled as in-service queue, and thus busy state can be cleared >> while reqs are not completed. >> >> Using bfq_del_bfqq_busy() will change behaviour in this specail case, >> do you think service guarantees will be broken? > > Well, I don't think so. Because slow disks don't tend to do a lot of > internal scheduling (or have deep IO queues for that matter). Also note > that generally bfq_select_queue() will not even expire a queue (despite it > not having any requests to dispatch) when we should not dispatch other > requests to maintain service guarantees. So I think service guarantees will > be generally preserved. Obviously I could be wrong, we we will not know > until we try it :). Thanks a lot for your explanation, I'll do some tests. And i'll send a new version if tests look good.