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=-10.2 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 20A00C433B4 for ; Sat, 17 Apr 2021 14:06:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E442461207 for ; Sat, 17 Apr 2021 14:06:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236058AbhDQOHW (ORCPT ); Sat, 17 Apr 2021 10:07:22 -0400 Received: from out30-132.freemail.mail.aliyun.com ([115.124.30.132]:59030 "EHLO out30-132.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233008AbhDQOHV (ORCPT ); Sat, 17 Apr 2021 10:07:21 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R211e4;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_---0UVqrXPK_1618668413; Received: from admindeMacBook-Pro-2.local(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0UVqrXPK_1618668413) by smtp.aliyun-inc.com(127.0.0.1); Sat, 17 Apr 2021 22:06:53 +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> From: JeffleXu Message-ID: <1fb6e15e-fb4d-a2bf-9f65-2ae2aa15a8a2@linux.alibaba.com> Date: Sat, 17 Apr 2021 22:06:53 +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 4/16/21 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? 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)) { 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). -- Thanks, Jeffle