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 E5396C43334 for ; Tue, 26 Jul 2022 03:21:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237509AbiGZDVv (ORCPT ); Mon, 25 Jul 2022 23:21:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34012 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230448AbiGZDVq (ORCPT ); Mon, 25 Jul 2022 23:21:46 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 926272A42C for ; Mon, 25 Jul 2022 20:21:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1658805704; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=UQtxuN/ySrkLbKK5noUUGeYHmVlLx996fbK2jBYrXpk=; b=jDAtNCpGEX6NGWZc7jsnZjR5kNM9lYlSYozq4U9ZU+ziEXia/BWCNQ8e1q+w+PLC+Eh3fR 570rIJkiDi1TEhJYeg3yDJnJfpwIliDzy4ukfbBImhjIgNXQmaztwvJ09cn1f1K9IeRlhr gu74JBp3aXMDPJA4PQP+8ATinKOUGiU= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-348-zG96v_E4P8K3Nih_QbVX4g-1; Mon, 25 Jul 2022 23:21:40 -0400 X-MC-Unique: zG96v_E4P8K3Nih_QbVX4g-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id CC0D7802E5D; Tue, 26 Jul 2022 03:21:39 +0000 (UTC) Received: from T590 (ovpn-8-27.pek2.redhat.com [10.72.8.27]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3BCB7492CA2; Tue, 26 Jul 2022 03:21:34 +0000 (UTC) Date: Tue, 26 Jul 2022 11:21:29 +0800 From: Ming Lei To: Yu Kuai Cc: Yu Kuai , Yufen Yu , axboe@kernel.dk, linux-block@vger.kernel.org, hch@lst.de, "zhangyi (F)" Subject: Re: [PATCH] blk-mq: run queue after issuing the last request of the plug list Message-ID: References: <0baa5b04-7194-54fa-08a5-51425601343e@huaweicloud.com> <6b070c7d-473a-cc96-def3-49826ca08aea@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <6b070c7d-473a-cc96-def3-49826ca08aea@huawei.com> X-Scanned-By: MIMEDefang 2.85 on 10.11.54.9 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Tue, Jul 26, 2022 at 11:14:23AM +0800, Yu Kuai wrote: > Hi, Ming > > 在 2022/07/26 11:02, Ming Lei 写道: > > On Tue, Jul 26, 2022 at 10:52:56AM +0800, Yu Kuai wrote: > > > Hi, Ming > > > 在 2022/07/26 10:32, Ming Lei 写道: > > > > On Tue, Jul 26, 2022 at 10:08:13AM +0800, Yu Kuai wrote: > > > > > 在 2022/07/26 9:46, Ming Lei 写道: > > > > > > On Tue, Jul 26, 2022 at 09:08:19AM +0800, Yu Kuai wrote: > > > > > > > Hi, Ming! > > > > > > > > > > > > > > 在 2022/07/25 23:43, Ming Lei 写道: > > > > > > > > On Sat, Jul 23, 2022 at 10:50:03AM +0800, Yu Kuai wrote: > > > > > > > > > Hi, Ming! > > > > > > > > > > > > > > > > > > 在 2022/07/19 17:26, Ming Lei 写道: > > > > > > > > > > On Mon, Jul 18, 2022 at 08:35:28PM +0800, Yufen Yu wrote: > > > > > > > > > > > We do test on a virtio scsi device (/dev/sda) and the default mq > > > > > > > > > > > scheduler is 'none'. We found a IO hung as following: > > > > > > > > > > > > > > > > > > > > > > blk_finish_plug > > > > > > > > > > > blk_mq_plug_issue_direct > > > > > > > > > > > scsi_mq_get_budget > > > > > > > > > > > //get budget_token fail and sdev->restarts=1 > > > > > > > > > > > > > > > > > > > > > > scsi_end_request > > > > > > > > > > > scsi_run_queue_async > > > > > > > > > > > //sdev->restart=0 and run queue > > > > > > > > > > > > > > > > > > > > > > blk_mq_request_bypass_insert > > > > > > > > > > > //add request to hctx->dispatch list > > > > > > > > > > > > > > > > > > > > Here the issue shouldn't be related with scsi's get budget or > > > > > > > > > > scsi_run_queue_async. > > > > > > > > > > > > > > > > > > > > If blk-mq adds request into ->dispatch_list, it is blk-mq core's > > > > > > > > > > responsibility to re-run queue for moving on. Can you investigate a > > > > > > > > > > bit more why blk-mq doesn't run queue after adding request to > > > > > > > > > > hctx dispatch list? > > > > > > > > > > > > > > > > > > I think Yufen is probably thinking about the following Concurrent > > > > > > > > > scenario: > > > > > > > > > > > > > > > > > > blk_mq_flush_plug_list > > > > > > > > > # assume there are three rq > > > > > > > > > blk_mq_plug_issue_direct > > > > > > > > > blk_mq_request_issue_directly > > > > > > > > > # dispatch rq1, succeed > > > > > > > > > blk_mq_request_issue_directly > > > > > > > > > # dispatch rq2 > > > > > > > > > __blk_mq_try_issue_directly > > > > > > > > > blk_mq_get_dispatch_budget > > > > > > > > > scsi_mq_get_budget > > > > > > > > > atomic_inc(&sdev->restarts); > > > > > > > > > # rq2 failed to get budget > > > > > > > > > # restarts is 1 now > > > > > > > > > scsi_end_request > > > > > > > > > # rq1 is completed > > > > > > > > > ┊scsi_run_queue_async > > > > > > > > > ┊ atomic_cmpxchg(&sdev->restarts, > > > > > > > > > old, 0) == old > > > > > > > > > ┊ # set restarts to 0 > > > > > > > > > ┊ blk_mq_run_hw_queues > > > > > > > > > ┊ # hctx->dispatch list is empty > > > > > > > > > blk_mq_request_bypass_insert > > > > > > > > > # insert rq2 to hctx->dispatch list > > > > > > > > > > > > > > > > After rq2 is added to ->dispatch_list in blk_mq_try_issue_list_directly(), > > > > > > > > no matter if list_empty(list) is empty or not, queue will be run either from > > > > > > > > blk_mq_request_bypass_insert() or blk_mq_sched_insert_requests(). > > > > > > > > > > > > > > 1) while inserting rq2 to dispatch list, blk_mq_request_bypass_insert() > > > > > > > is called from blk_mq_try_issue_list_directly(), list_empty() won't > > > > > > > pass, thus thus blk_mq_request_bypass_insert() won't run queue. > > > > > > > > > > > > Yeah, but in blk_mq_try_issue_list_directly() after rq2 is inserted to dispatch > > > > > > list, the loop is broken and blk_mq_try_issue_list_directly() returns to > > > > > > blk_mq_sched_insert_requests() in which list_empty() is false, so > > > > > > blk_mq_insert_requests() and blk_mq_run_hw_queue() are called, queue > > > > > > is still run. > > > > > > > > > > > > Also not sure why you make rq3 involved, since the list is local list on > > > > > > stack, and it can be operated concurrently. > > > > > > > > > > I make rq3 involved because there are some conditions that > > > > > blk_mq_insert_requests() and blk_mq_run_hw_queue() won't be called from > > > > > blk_mq_sched_insert_requests(): > > > > > > > > The two won't be called if list_empty() is true, and will be called if > > > > !list_empty(). > > > > > > > > That is why I mentioned run queue has been done after rq2 is added to > > > > ->dispatch_list. > > > > > > I don't follow here, it's right after rq2 is inserted to dispatch list, > > > list is not empty, and blk_mq_sched_insert_requests() will be called. > > > However, do you think that it's impossible that > > > blk_mq_sched_insert_requests() can dispatch rq in the list and list > > > will become empty? > > > > Please take a look at blk_mq_sched_insert_requests(). > > > > When codes runs into blk_mq_sched_insert_requests(), the following > > blk_mq_run_hw_queue() will be run always, how does list empty or not > > make a difference there? > > This is strange, always blk_mq_run_hw_queue() is exactly what Yufen > tries to do in this patch, are we look at different code? No. > > I'm copying blk_mq_sched_insert_requests() here, the code is from > latest linux-next: > > 461 void blk_mq_sched_insert_requests(struct blk_mq_hw_ctx *hctx, > 462 ┊ struct blk_mq_ctx *ctx, > 463 ┊ struct list_head *list, bool > run_queue_async) > 464 { > 465 struct elevator_queue *e; > 466 struct request_queue *q = hctx->queue; > 467 > 468 /* > 469 ┊* blk_mq_sched_insert_requests() is called from flush plug > 470 ┊* context only, and hold one usage counter to prevent queue > 471 ┊* from being released. > 472 ┊*/ > 473 percpu_ref_get(&q->q_usage_counter); > 474 > 475 e = hctx->queue->elevator; > 476 if (e) { > 477 e->type->ops.insert_requests(hctx, list, false); > 478 } else { > 479 /* > 480 ┊* try to issue requests directly if the hw queue isn't > 481 ┊* busy in case of 'none' scheduler, and this way may > save > 482 ┊* us one extra enqueue & dequeue to sw queue. > 483 ┊*/ > 484 if (!hctx->dispatch_busy && !run_queue_async) { > 485 blk_mq_run_dispatch_ops(hctx->queue, > 486 blk_mq_try_issue_list_directly(hctx, > list)); > 487 if (list_empty(list)) > 488 goto out; > 489 } > 490 blk_mq_insert_requests(hctx, ctx, list); > 491 } > 492 > 493 blk_mq_run_hw_queue(hctx, run_queue_async); > 494 out: > 495 percpu_ref_put(&q->q_usage_counter); > 496 } > > Here in line 487, if list_empty() is true, out label will skip > run_queue(). If list_empty() is true, run queue is guaranteed to run in blk_mq_try_issue_list_directly() in case that BLK_STS_*RESOURCE is returned from blk_mq_request_issue_directly(). ret = blk_mq_request_issue_directly(rq, list_empty(list)); if (ret != BLK_STS_OK) { if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE) { blk_mq_request_bypass_insert(rq, false, list_empty(list)); //run queue break; } blk_mq_end_request(rq, ret); errors++; } else queued++; So why do you try to add one extra run queue? Thanks, Ming