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 CACAFC433EF for ; Tue, 26 Jul 2022 01:47:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236844AbiGZBrS (ORCPT ); Mon, 25 Jul 2022 21:47:18 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41322 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229853AbiGZBrS (ORCPT ); Mon, 25 Jul 2022 21:47:18 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 0622A2873B for ; Mon, 25 Jul 2022 18:47:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1658800036; 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=XT0+7+veTqwb8/7O26bzSt+Ko+fpNBXGSok/p4K98o0=; b=fSIZoYFxX+7gFs9tWjfQ3qSUYvkMG7HYSYv32LtPBFlZKsEcoSUbes3mB2yat9YQQhY6h7 w8u85tEd0n0Ph4BTasPZFO8zJWoHogaDcJxeLZx9u4XsMaj6YZGJ89aB0CAR7aa4GqYpg/ 0/iXSqlFJwPoLpv62blQi6fq3UofsHg= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-491-YYIIv9nZNMahP9GfVtROsg-1; Mon, 25 Jul 2022 21:47:09 -0400 X-MC-Unique: YYIIv9nZNMahP9GfVtROsg-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 31B6F280F2A5; Tue, 26 Jul 2022 01:47:09 +0000 (UTC) Received: from T590 (ovpn-8-27.pek2.redhat.com [10.72.8.27]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C7881492CA2; Tue, 26 Jul 2022 01:47:02 +0000 (UTC) Date: Tue, 26 Jul 2022 09:46:56 +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: <20220718123528.178714-1-yuyufen@huawei.com> <0baa5b04-7194-54fa-08a5-51425601343e@huaweicloud.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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 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. Thanks, Ming