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 CC95FC6FA82 for ; Fri, 23 Sep 2022 10:23:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230410AbiIWKXN (ORCPT ); Fri, 23 Sep 2022 06:23:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42680 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229512AbiIWKXJ (ORCPT ); Fri, 23 Sep 2022 06:23:09 -0400 Received: from dggsgout11.his.huawei.com (unknown [45.249.212.51]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 296B0131F5E; Fri, 23 Sep 2022 03:23:07 -0700 (PDT) Received: from mail02.huawei.com (unknown [172.30.67.143]) by dggsgout11.his.huawei.com (SkyGuard) with ESMTP id 4MYp8f4CXKzl83x; Fri, 23 Sep 2022 18:21:22 +0800 (CST) Received: from [10.174.176.73] (unknown [10.174.176.73]) by APP2 (Coremail) with SMTP id Syh0CgAnenMHiS1jJq8VBQ--.59256S3; Fri, 23 Sep 2022 18:23:05 +0800 (CST) Subject: Re: [PATCH v3 3/5] block, bfq: don't disable wbt if CONFIG_BFQ_GROUP_IOSCHED is disabled To: Jan Kara , Yu Kuai Cc: Christoph Hellwig , paolo.valente@linaro.org, axboe@kernel.dk, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, yi.zhang@huawei.com, "yukuai (C)" References: <20220922113558.1085314-1-yukuai3@huawei.com> <20220922113558.1085314-4-yukuai3@huawei.com> <988a86f2-e960-ba59-4d41-f4c8a6345ee9@huaweicloud.com> <20220923100659.a3atdanlvygffuxt@quack3> From: Yu Kuai Message-ID: <95998ae6-8bbf-b438-801b-7033ceaf9c36@huaweicloud.com> Date: Fri, 23 Sep 2022 18:23:03 +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: <20220923100659.a3atdanlvygffuxt@quack3> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit X-CM-TRANSID: Syh0CgAnenMHiS1jJq8VBQ--.59256S3 X-Coremail-Antispam: 1UD129KBjvJXoW7WF45tFW8WFy7GrykZF1DZFb_yoW8ur1Up3 yfWayIkF4rAFWxKwnFy3y8Jryrtws7Jr45WF1rCrZ7Cas8tr1xGw1fGF4Y9a4Uur18Gw12 yF4rXrZ7Ca4DZaDanT9S1TB71UUUUUUqnTZGkaVYY2UrUUUUjbIjqfuFe4nvWSU5nxnvy2 9KBjDU0xBIdaVrnRJUUUkE14x267AKxVW8JVW5JwAFc2x0x2IEx4CE42xK8VAvwI8IcIk0 rVWrJVCq3wAFIxvE14AKwVWUJVWUGwA2ocxC64kIII0Yj41l84x0c7CEw4AK67xGY2AK02 1l84ACjcxK6xIIjxv20xvE14v26w1j6s0DM28EF7xvwVC0I7IYx2IY6xkF7I0E14v26r4U JVWxJr1l84ACjcxK6I8E87Iv67AKxVW0oVCq3wA2z4x0Y4vEx4A2jsIEc7CjxVAFwI0_Gc CE3s1le2I262IYc4CY6c8Ij28IcVAaY2xG8wAqx4xG64xvF2IEw4CE5I8CrVC2j2WlYx0E 2Ix0cI8IcVAFwI0_Jr0_Jr4lYx0Ex4A2jsIE14v26r1j6r4UMcvjeVCFs4IE7xkEbVWUJV W8JwACjcxG0xvEwIxGrwACjI8F5VA0II8E6IAqYI8I648v4I1lc7I2V7IY0VAS07AlzVAY IcxG8wCF04k20xvY0x0EwIxGrwCFx2IqxVCFs4IE7xkEbVWUJVW8JwC20s026c02F40E14 v26r1j6r18MI8I3I0E7480Y4vE14v26r106r1rMI8E67AF67kF1VAFwI0_Jw0_GFylIxkG c2Ij64vIr41lIxAIcVC0I7IYx2IY67AKxVWUJVWUCwCI42IY6xIIjxv20xvEc7CjxVAFwI 0_Jr0_Gr1lIxAIcVCF04k26cxKx2IYs7xG6rWUJVWrZr1UMIIF0xvEx4A2jsIE14v26r1j 6r4UMIIF0xvEx4A2jsIEc7CjxVAFwI0_Gr0_Gr1UYxBIdaVFxhVjvjDU0xZFpf9x0JUdHU DUUUUU= X-CM-SenderInfo: 51xn3trlr6x35dzhxuhorxvhhfrp/ X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Hi, Jan 在 2022/09/23 18:06, Jan Kara 写道: > On Fri 23-09-22 17:50:49, Yu Kuai wrote: >> Hi, Christoph >> >> 在 2022/09/23 16:56, Christoph Hellwig 写道: >>> On Thu, Sep 22, 2022 at 07:35:56PM +0800, Yu Kuai wrote: >>>> wbt and bfq should work just fine if CONFIG_BFQ_GROUP_IOSCHED is disabled. >>> >>> Umm, wouldn't this be something decided at runtime, that is not >>> if CONFIG_BFQ_GROUP_IOSCHED is enable/disable in the kernel build >>> if the hierarchical cgroup based scheduling is actually used for a >>> given device? >>> . >>> >> >> That's a good point, >> >> Before this patch wbt is simply disabled if elevator is bfq. >> >> With this patch, if elevator is bfq while bfq doesn't throttle >> any IO yet, wbt still is disabled unnecessarily. > > It is not really disabled unnecessarily. Have you actually tested the > performance of the combination? I did once and the results were just > horrible (which is I made BFQ just disable wbt by default). The problem is > that blk-wbt assumes certain model of underlying storage stack and hardware > behavior and BFQ just does not fit in that model. For example BFQ wants to > see as many requests as possible so that it can heavily reorder them, > estimate think times of applications, etc. On the other hand blk-wbt > assumes that if request latency gets higher, it means there is too much IO > going on and we need to allow less of "lower priority" IO types to be > submitted. These two go directly against one another and I was easily > observing blk-wbt spiraling down to allowing only very small number of > requests submitted while BFQ was idling waiting for more IO from the > process that was currently scheduled. > Thanks for your explanation, I understand that bfq and wbt should not work together. However, I wonder if CONFIG_BFQ_GROUP_IOSCHED is disabled, or service guarantee is not needed, does the above phenomenon still exist? I find it hard to understand... Perhaps I need to do some test. Thanks, Kuai > So I'm kind of wondering why you'd like to use blk-wbt and BFQ together... > > Honza >