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 X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3D19AC433B4 for ; Tue, 20 Apr 2021 07:25:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 062A460238 for ; Tue, 20 Apr 2021 07:25:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229538AbhDTHZ4 (ORCPT ); Tue, 20 Apr 2021 03:25:56 -0400 Received: from out30-133.freemail.mail.aliyun.com ([115.124.30.133]:40372 "EHLO out30-133.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229527AbhDTHZz (ORCPT ); Tue, 20 Apr 2021 03:25:55 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R481e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04400;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=5;SR=0;TI=SMTPD_---0UWBJ9g1_1618903521; Received: from admindeMacBook-Pro-2.local(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0UWBJ9g1_1618903521) by smtp.aliyun-inc.com(127.0.0.1); Tue, 20 Apr 2021 15:25:22 +0800 Subject: Re: [PATCH] block: introduce QUEUE_FLAG_POLL_CAP flag To: Ming Lei Cc: snitzer@redhat.com, axboe@kernel.dk, linux-block@vger.kernel.org, dm-devel@redhat.com References: <20210401021927.343727-12-ming.lei@redhat.com> <20210416080037.26335-1-jefflexu@linux.alibaba.com> <1fb6e15e-fb4d-a2bf-9f65-2ae2aa15a8a2@linux.alibaba.com> From: JeffleXu Message-ID: <0b762419-4c46-1e31-d97e-4ce2abc6a91c@linux.alibaba.com> Date: Tue, 20 Apr 2021 15:25:21 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 4/19/21 9:36 PM, Ming Lei wrote: > On Mon, Apr 19, 2021 at 01:40:21PM +0800, JeffleXu wrote: >> >> >> On 4/19/21 10:21 AM, Ming Lei wrote: >>> On Sat, Apr 17, 2021 at 10:06:53PM +0800, JeffleXu wrote: >>>> >>>> >>>> On 4/16/21 5:07 PM, Ming Lei wrote: >>>>> On Fri, Apr 16, 2021 at 04:00:37PM +0800, Jeffle Xu wrote: >>>>>> Hi, >>>>>> How about this patch to remove the extra poll_capable() method? >>>>>> >>>>>> And the following 'dm: support IO polling for bio-based dm device' needs >>>>>> following change. >>>>>> >>>>>> ``` >>>>>> + /* >>>>>> + * Check for request-based device is remained to >>>>>> + * dm_mq_init_request_queue()->blk_mq_init_allocated_queue(). >>>>>> + * For bio-based device, only set QUEUE_FLAG_POLL when all underlying >>>>>> + * devices supporting polling. >>>>>> + */ >>>>>> + if (__table_type_bio_based(t->type)) { >>>>>> + if (dm_table_supports_poll(t)) { >>>>>> + blk_queue_flag_set(QUEUE_FLAG_POLL_CAP, q); >>>>>> + blk_queue_flag_set(QUEUE_FLAG_POLL, q); >>>>>> + } >>>>>> + else { >>>>>> + blk_queue_flag_clear(QUEUE_FLAG_POLL, q); >>>>>> + blk_queue_flag_clear(QUEUE_FLAG_POLL_CAP, q); >>>>>> + } >>>>>> + } >>>>>> ``` >>>>> >>>>> Frankly speaking, I don't see any value of using QUEUE_FLAG_POLL_CAP for >>>>> DM, and the result is basically subset of treating DM as always being capable >>>>> of polling. >>>>> >>>>> Also underlying queue change(either limits or flag) won't be propagated >>>>> to DM/MD automatically. Strictly speaking it doesn't matter if all underlying >>>>> queues are capable of supporting polling at the exact time of 'write sysfs/poll', >>>>> cause any of them may change in future. >>>>> >>>>> So why not start with the simplest approach(always capable of polling) >>>>> which does meet normal bio based polling requirement? >>>>> >>>> >>>> I find one scenario where this issue may matter. Consider the scenario >>>> where HIPRI bios are submitted to DM device though **all** underlying >>>> devices has been disabled for polling. In this case, a **valid** cookie >>>> (pid of current submitting process) is still returned. Then if @spin of >>>> the following blk_poll() is true, blk_poll() will get stuck in dead loop >>>> because blk_mq_poll() always returns 0, since previously submitted bios >>>> are all enqueued into IRQ hw queue. >>>> >>>> Maybe you need to re-remove the bio from the poll context if the >>>> returned cookie is BLK_QC_T_NONE? >>> >>> It won't be one issue, see blk_bio_poll_preprocess() which is called >>> from submit_bio_checks(), so any bio's HIPRI will be cleared if the >>> queue doesn't support POLL, that code does cover underlying bios. >> >> Sorry there may be some confusion in my description. Let's discuss in >> the following scenario: MD/DM advertise QUEUE_FLAG_POLL, though **all** >> underlying devices are without QUEUE_FLAG_POLL. This scenario is >> possible, if you want to enable MD/DM's polling without checking the >> capability of underlying devices. >> >> In this case, it seems that REQ_HIPRI is kept for both MD/DM and >> underlying blk-mq devices. I used to think that REQ_HIPRI will be >> cleared for underlying blk-mq deivces, but now it seems that REQ_HIPRI >> of bios submitted to underlying blk-mq deivces won't be cleared, since >> submit_bio_checks() is only called in the entry of submit_bio(), not in >> the while() loop of __submit_bio_noacct_ctx(). Though these underlying >> blk-mq devices don't support IO polling at all, or they all have been >> disabled for polling, REQ_HIPRI bios are finally submitted down. >> >> Or do I miss something? > > No matter the loop, the bios are actually submitted to the > current->bio_list via submit_bio_noacct() or submit_bio(). > 'grep -r submit_bio drivers/md' will show you the point. Oops. I forgot that. Thanks and sorry for the noise. So if that's the case, it seems that patch 11/12 are not needed anymore. -- Thanks, Jeffle