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=-1.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_PASS 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 95ACAC43381 for ; Tue, 2 Apr 2019 02:02:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5A77420830 for ; Tue, 2 Apr 2019 02:02:49 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="dJeuKAjb" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726841AbfDBCCs (ORCPT ); Mon, 1 Apr 2019 22:02:48 -0400 Received: from userp2120.oracle.com ([156.151.31.85]:48656 "EHLO userp2120.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726269AbfDBCCs (ORCPT ); Mon, 1 Apr 2019 22:02:48 -0400 Received: from pps.filterd (userp2120.oracle.com [127.0.0.1]) by userp2120.oracle.com (8.16.0.27/8.16.0.27) with SMTP id x321wcnt156302; Tue, 2 Apr 2019 02:02:36 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=subject : to : cc : references : from : message-id : date : mime-version : in-reply-to : content-type : content-transfer-encoding; s=corp-2018-07-02; bh=GNEYHlj+pXEJT3D/K85vUmRNvxkaMAelc2AUab3NE+k=; b=dJeuKAjbHae2YtSp88N8h6a4Bpd8JEc10WhqaacOlrEKjtgH7xABwVAdFJMjv7UO0Q68 WRF327kYfwiXgYNl49HNeTGYbsTXJxqK5QnEFGKHYdQExxYBc2e0neYd+GSqWlU/Wnuo FtNJ1fP0HUYfEY1SKq1sN7WDM909ojNhUnPIFGJRQ1NOtnmSyDR7kB6gSTbrxnewGHay XePC8FZzreUz84vQAI9jSePom2Y3tRKVCWGkDOf+0viheHYCxlBNpVTEfX59grx/63+N Che7j1n2Fo63u3UJ9xlAJWMSCa8diAPK+tdvyvI4mqtyR/VsGXDjzyRHSQp69Lb8NOVJ uQ== Received: from userv0022.oracle.com (userv0022.oracle.com [156.151.31.74]) by userp2120.oracle.com with ESMTP id 2rj13q28ex-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 02 Apr 2019 02:02:36 +0000 Received: from aserv0122.oracle.com (aserv0122.oracle.com [141.146.126.236]) by userv0022.oracle.com (8.14.4/8.14.4) with ESMTP id x3222Yhp008584 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 2 Apr 2019 02:02:35 GMT Received: from abhmp0005.oracle.com (abhmp0005.oracle.com [141.146.116.11]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id x3222X0V008052; Tue, 2 Apr 2019 02:02:34 GMT Received: from [10.182.71.8] (/10.182.71.8) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Mon, 01 Apr 2019 19:02:33 -0700 Subject: Re: [PATCH 0/5] blk-mq: allow to run queue if queue refcount is held To: Ming Lei Cc: Bart Van Assche , Jens Axboe , linux-block@vger.kernel.org, James Smart , Bart Van Assche , linux-scsi@vger.kernel.org, "Martin K . Petersen" , Christoph Hellwig , "James E . J . Bottomley" References: <20190331030954.22320-1-ming.lei@redhat.com> <10c8ed10-3c96-b73c-18d8-114773b1d675@acm.org> <20190401020036.GB30776@ming.t460p> <20190401025237.GE30776@ming.t460p> <21b2000b-16b6-f5a6-692b-73143a49a4ec@oracle.com> <20190401032852.GG30776@ming.t460p> <20190401100334.GA5493@ming.t460p> From: "jianchao.wang" Message-ID: Date: Tue, 2 Apr 2019 10:02:43 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1 MIME-Version: 1.0 In-Reply-To: <20190401100334.GA5493@ming.t460p> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9214 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1904020013 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Hi Ming On 4/1/19 6:03 PM, Ming Lei wrote: > On Mon, Apr 01, 2019 at 05:19:01PM +0800, jianchao.wang wrote: >> Hi Ming >> >> On 4/1/19 11:28 AM, Ming Lei wrote: >>> On Mon, Apr 01, 2019 at 11:25:50AM +0800, jianchao.wang wrote: >>>> Hi Ming >>>> >>>> On 4/1/19 10:52 AM, Ming Lei wrote: >>>>>> percpu_ref_tryget_live() fails if a per-cpu counter is in the "dead" state. >>>>>> percpu_ref_kill() changes the state of a per-cpu counter to the "dead" >>>>>> state. blk_freeze_queue_start() calls percpu_ref_kill(). blk_cleanup_queue() >>>>>> already calls blk_set_queue_dying() and that last function calls >>>>>> blk_freeze_queue_start(). So I think that what you wrote is not correct and >>>>>> that inserting a percpu_ref_tryget_live()/percpu_ref_put() pair in >>>>>> blk_mq_run_hw_queues() or blk_mq_run_hw_queue() would make a difference and >>>>>> also that moving the percpu_ref_exit() call into blk_release_queue() makes >>>>>> sense. >>>>> If percpu_ref_exit() is moved to blk_release_queue(), we still need to >>>>> move freeing of hw queue's resource into blk_release_queue() like what >>>>> the patchset is doing. >>>>> >>>>> Then we don't need to get/put q_usage_counter in blk_mq_run_hw_queues() any more, >>>>> do we? >>>> >>>> IMO, if we could get a way to prevent any attempt to run queue, it would be >>>> better and clearer. >>> >>> It is hard to do that way, and not necessary. >>> >>> I will post V2 soon for review. >>> >> >> Put percpu_ref_tryget/put pair into blk_mq_run_hw_queues could stop run queue after >> requet_queue is frozen and drained (run queue is also unnecessary because there is no >> entered requests). And also percpu_ref_tryget could avoid the io hung issue you mentioned. >> We have similar one in blk_mq_timeout_work. > > If percpu_ref_tryget() is used, percpu_ref_exit() has to be moved into > queue's release handler. > > Then we still have to move freeing hctx's resource into hctx or queue's > release handler, that is exactly what this patch is doing. Then > percpu_ref_tryget() becomes unnecessary again, right? I'm not sure about the percpu_ref_exit. Perhaps I have some misunderstanding about it. >From the code of it, it frees the percpu_count and set ref->percpu_count_ptr to __PERCPU_REF_ATOMIC_DEAD. The comment says 'the caller is responsible for ensuring that @ref is no longer in active use' But if we use it after kill, does it count a active use ? Based on the code, the __ref_is_percpu is always false during this, and percpu_ref_tryget will not touch the freed percpu counter but just the atomic ref->count. It looks safe. > >> >> freeze and drain queue to stop new attempt to run queue, blk_sync_queue syncs and stops >> the started ones, then hctx->run_queue is cleaned totally. >> >> IMO, it would be better to have a checkpoint after which there will be no any in-flight >> asynchronous activities of the request_queue (hctx->run_work, q->requeue_work, q-> timeout_work) >> and any attempt to start them will fail. > > All are canceled in blk_cleanup_queue(), but not enough, given queue can > be run in sync mode(such as via plug, direct issue, ...), or driver's > requeue, such as SCSI's requeue. SCSI's requeue may run other LUN's queue > just by holding queue's kobject refcount. Yes, so we need a checkpoint here to ensure the request_queue to enter into a certain state. We provide a guarantee that all of the activities are stopped after this checkpoint. It will be convenient for us to do other things following, for example release request_queue's resource. Thanks Jianchao > >> >> Perhaps, this will be a good change to do this ；） > > However, I don't see it is necessary if we simply move freeing hctx's > resource into its release handler, just like V2. > > > Thanks, > Ming >