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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=ham 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 AFF45C433B4 for ; Mon, 19 Apr 2021 05:40:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7E4BD6113C for ; Mon, 19 Apr 2021 05:40:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231714AbhDSFkz (ORCPT ); Mon, 19 Apr 2021 01:40:55 -0400 Received: from out30-54.freemail.mail.aliyun.com ([115.124.30.54]:48600 "EHLO out30-54.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230022AbhDSFkx (ORCPT ); Mon, 19 Apr 2021 01:40:53 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R851e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04426;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=5;SR=0;TI=SMTPD_---0UVzpdf9_1618810821; Received: from admindeMacBook-Pro-2.local(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0UVzpdf9_1618810821) by smtp.aliyun-inc.com(127.0.0.1); Mon, 19 Apr 2021 13:40:22 +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: Date: Mon, 19 Apr 2021 13:40:21 +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 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? > >> >> >> Something like: >> >> -static blk_qc_t __submit_bio_noacct(struct bio *bio) >> +static blk_qc_t __submit_bio_noacct_ctx(struct bio *bio, struct >> io_context *ioc) >> { >> struct bio_list bio_list_on_stack[2]; >> blk_qc_t ret = BLK_QC_T_NONE; >> @@ -1047,7 +1163,15 @@ static blk_qc_t __submit_bio_noacct(struct bio *bio) >> bio_list_on_stack[1] = bio_list_on_stack[0]; >> bio_list_init(&bio_list_on_stack[0]); >> >> if (ioc && queue_is_mq(q) && (bio->bi_opf & REQ_HIPRI)) { > > REQ_HIPRI won't be set for underlying bios which queue doesn't support > poll, so this branch won't be reached. Sorry I missed the '(bio->bi_opf & REQ_HIPRI)' condition here. Indeed bio without REQ_HIPRI won't be enqueued into the poll_context. > And the submission queue will > be empty, and blk_poll() for DM/MD(bio based queue) checks nothing, but > the polling won't be stopped until the iocb is completed. And this > handling is actually same with current polling on IRQ based queue. > >> bool queued = blk_bio_poll_prep_submit(ioc, bio); >> >> ret = __submit_bio(bio); >> + if (queued && !blk_qc_t_valid(ret)) >> /* TODO:remove bio from poll_context */ >> >> bio_set_private_data(bio, ret); >> } else { >> ret = __submit_bio(bio); >> } >> >> >> Then if you'd like fix this in this way, the returned value of >> .submit_bio() of DM/MD also needs to return BLK_QC_T_NONE now. Currently >> .submit_bio() of DM actually returns the cookie of the last split bio >> (to underlying mq deivice). > > I am a bit confused, this patch requires .submit_bio() of DM/MD(bio > based queue) to return either 0 or pid of the submission task. > > > Thanks, > Ming > -- Thanks, Jeffle