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=-5.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,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 E7547C433E0 for ; Fri, 19 Jun 2020 22:24:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C1FFF222E8 for ; Fri, 19 Jun 2020 22:24:20 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Lmh72bNP" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729337AbgFSWYU (ORCPT ); Fri, 19 Jun 2020 18:24:20 -0400 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:58472 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1729229AbgFSWYT (ORCPT ); Fri, 19 Jun 2020 18:24:19 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1592605458; 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: in-reply-to:in-reply-to:references:references; bh=Tmjs5D6L522iN3BsMMMDQ2azzXRy6e+DKSTe5FLnkR0=; b=Lmh72bNPn+PQ3Xh85E8plJm/XF2TFXg0IABYgJVpfOdIt6qrQOWyLdM/10PhAHH67hAx6/ 24uRCF6eRuszz8cbXKNsJOkb8ig89VgAMtZdpG+fweek2rYVHOiAvQOQPy2SIR6/lDK444 TnoJvaz1IxJ3CvborEEK+gxcowhabXM= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-105-Jc7efI1aO5ep3XIPql9eFw-1; Fri, 19 Jun 2020 18:24:16 -0400 X-MC-Unique: Jc7efI1aO5ep3XIPql9eFw-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 16FBD835B40; Fri, 19 Jun 2020 22:24:15 +0000 (UTC) Received: from T590 (ovpn-12-19.pek2.redhat.com [10.72.12.19]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0ADDE5D9E5; Fri, 19 Jun 2020 22:24:03 +0000 (UTC) Date: Sat, 20 Jun 2020 06:23:59 +0800 From: Ming Lei To: Mike Snitzer Cc: dm-devel@redhat.com, linux-block@vger.kernel.org, axboe@kernel.dk Subject: Re: dm-rq: don't call blk_mq_queue_stopped in dm_stop_queue() Message-ID: <20200619222359.GA353853@T590> References: <20200619084214.337449-1-ming.lei@redhat.com> <20200619094250.GA18410@redhat.com> <20200619101142.GA339442@T590> <20200619160657.GA24520@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200619160657.GA24520@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Fri, Jun 19, 2020 at 12:06:57PM -0400, Mike Snitzer wrote: > On Fri, Jun 19 2020 at 6:11am -0400, > Ming Lei wrote: > > > Hi Mike, > > > > On Fri, Jun 19, 2020 at 05:42:50AM -0400, Mike Snitzer wrote: > > > Hi Ming, > > > > > > Thanks for the patch! But I'm having a hard time understanding what > > > you've written in the patch header, > > > > > > On Fri, Jun 19 2020 at 4:42am -0400, > > > Ming Lei wrote: > > > > > > > dm-rq won't stop queue, meantime blk-mq won't stop one queue too, so > > > > remove the check. > > > > > > It'd be helpful if you could unpack this with more detail before going on > > > to explain why using blk_queue_quiesced, despite dm-rq using > > > blk_mq_queue_stopped, would also be ineffective. > > > > > > SO: > > > > > > > dm-rq won't stop queue > > > > > > 1) why won't dm-rq stop the queue? Do you mean it won't reliably > > > _always_ stop the queue because of the blk_mq_queue_stopped() check? > > > > device mapper doesn't call blk_mq_stop_hw_queue or blk_mq_stop_hw_queues. > > > > > > > > > meantime blk-mq won't stop one queue too, so remove the check. > > > > > > 2) Meaning?: blk_mq_queue_stopped() will return true even if only one hw > > > queue is stopped, given blk-mq must stop all hw queues a positive return > > > from this blk_mq_queue_stopped() check is incorrectly assuming it meanss > > > all hw queues are stopped. > > > > blk-mq won't call blk_mq_stop_hw_queue or blk_mq_stop_hw_queues for > > dm-rq's queue too, so dm-rq's hw queue won't be stopped. > > > > BTW blk_mq_stop_hw_queue or blk_mq_stop_hw_queues are supposed to be > > used for throttling queue. > > I'm going to look at actually stopping the queue (using one of these > interfaces). I didn't realize I wasn't actually stopping the queue. > The intent was to do so. > > In speaking with Jens yesterday about freeze vs stop: it is clear that > dm-rq needs to still be able to allocate new requests, but _not_ call > the queue_rq to issue the requests, while "stopped" (due to dm-mpath > potentially deferring retries of failed requests because of path failure > while quiescing the queue during DM device suspend). But that freezing > the queue goes too far because it won't allow such request allocation. Freezing shouldn't be a good choice for driver usually, and quiesce is exactly what you expect: request allocation is allowed, meantime, no .queue_rq is possible. > > > > > dm_stop_queue() actually tries to quiesce hw queues via blk_mq_quiesce_queue(), > > > > we can't check via blk_queue_quiesced for avoiding unnecessary queue > > > > quiesce because the flag is set before synchronize_rcu() and dm_stop_queue > > > > may be called when synchronize_rcu from another blk_mq_quiesce_queue is > > > > in-progress. > > > > > > But I'm left with questions/confusion on this too: > > > > > > 1) you mention blk_queue_quiesced instead of blk_mq_queue_stopped, so I > > > assume you mean that: not only is blk_mq_queue_stopped() > > > ineffective, blk_queue_quiesced() would be too? > > > > blk_mq_queue_stopped isn't necessary because dm-rq's hw queue won't be > > stopped by anyone, meantime replacing it with blk_queue_quiesced() is wrong. > > > > > > > > 2) the race you detail (with competing blk_mq_quiesce_queue) relative to > > > synchronize_rcu() and testing "the flag" is very detailed yet vague. > > > > If two code paths are calling dm_stop_queue() at the same time, one path may > > return immediately and it is wrong, sine synchronize_rcu() from another path > > may not be done. > > > > > > > > Anyway, once we get this heaader cleaned up a bit more I'll be happy to > > > get this staged as a stable@ fix for 5.8 inclusion ASAP. > > > > This patch isn't a fix, and it shouldn't be related with rhel8's issue. > > I realize that now. I've changed the patch header to be a bit clearer > and staged it for 5.9, see: > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.9&id=06e788ed59e0095b679bdce9e39c1a251032ae62 Thanks! -- Ming