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=-0.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no 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 47BDFC433E0 for ; Fri, 19 Jun 2020 22:38:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 20988222C8 for ; Fri, 19 Jun 2020 22:38:08 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="FeWdyNFQ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729422AbgFSWiH (ORCPT ); Fri, 19 Jun 2020 18:38:07 -0400 Received: from us-smtp-1.mimecast.com ([207.211.31.81]:37841 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1729229AbgFSWiH (ORCPT ); Fri, 19 Jun 2020 18:38:07 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1592606285; 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=eKuitHgDPYShTwVDBSPWX6rDgM+R9fFbQxKkuH+aFJY=; b=FeWdyNFQ17N7ldkslMu6y4STA//NuBAR66XadVdGWf98w1eDcudlE9xTLbum80i5pJ1u+d jIG1XPtwmT2adGUPKb4Vtc9OkcQCsTNUf4T8QPFL1VktzfWOD69f6dFc4nLUA3LVnP106d tDp2vKuDE9CLKnV4ujGDWpZpnNw3irg= 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-229-xcoEw7QqMhuIsnHFLK6Pqg-1; Fri, 19 Jun 2020 18:38:01 -0400 X-MC-Unique: xcoEw7QqMhuIsnHFLK6Pqg-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id D1CA3EC1A0; Fri, 19 Jun 2020 22:38:00 +0000 (UTC) Received: from T590 (ovpn-12-19.pek2.redhat.com [10.72.12.19]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D9BEA5BAD1; Fri, 19 Jun 2020 22:37:49 +0000 (UTC) Date: Sat, 20 Jun 2020 06:37:44 +0800 From: Ming Lei To: Mike Snitzer Cc: dm-devel@redhat.com, linux-block@vger.kernel.org, axboe@kernel.dk, Bart Van Assche Subject: Re: dm-rq: don't call blk_mq_queue_stopped in dm_stop_queue() Message-ID: <20200619223744.GB353853@T590> References: <20200619084214.337449-1-ming.lei@redhat.com> <20200619094250.GA18410@redhat.com> <20200619101142.GA339442@T590> <20200619160657.GA24520@redhat.com> <20200619174040.GA24968@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200619174040.GA24968@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 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 01:40:41PM -0400, Mike Snitzer wrote: > On Fri, Jun 19 2020 at 12:06pm -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. > > Seems I'm damned if I do (stop) or damned if I don't (new reports of > requests completing after DM device suspend's > blk_mq_quiesce_queue()+dm_wait_for_completion()). request(but not new) completing is possible after blk_mq_quiesce_queue()+ dm_wait_for_completion, because blk_mq_rq_inflight() only checks INFLIGHT request. If all requests are marked as MQ_RQ_COMPLETE, blk_mq_rq_inflight() still may return false. However, MQ_RQ_COMPLETE is one transient state. So what does dm-rq expect from dm_wait_for_completion()? If it is just no new request entering dm_queue_rq(), there shouldn't be issue. If dm-rq hopes there aren't any real inflight request(MQ_RQ_COMPLETE & MQ_RQ_INFLIGHT), we can change blk_mq_rq_inflight to support that. Thanks, Ming