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=-4.1 required=3.0 tests=BAYES_00,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 09B3EC433E0 for ; Fri, 31 Jul 2020 00:28:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D9C782082E for ; Fri, 31 Jul 2020 00:28:58 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="hdpJ5tEq" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730834AbgGaA25 (ORCPT ); Thu, 30 Jul 2020 20:28:57 -0400 Received: from us-smtp-2.mimecast.com ([205.139.110.61]:38780 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730800AbgGaA24 (ORCPT ); Thu, 30 Jul 2020 20:28:56 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1596155335; 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=9OWTPiMfSRJxshvx6fPA2F7HS2YyXzXf7Obr9RHhwHY=; b=hdpJ5tEqYM1g4tX069RdpgMQu8r6wbxrG+QYjaP13aeyLrBSD2/+heqecXKIbXWXuOEFnR i9OueTiiRNWn42brsjMm2Z31qUo6XJTTGgeAcqvupshv8MKQIZBbb/acVSGMgj9onqOr9m bkAMCy2U7FROBOTF6w6URSHXRv+r7NI= 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-280-_s1wQzlfPm6ORVb8396lFQ-1; Thu, 30 Jul 2020 20:28:50 -0400 X-MC-Unique: _s1wQzlfPm6ORVb8396lFQ-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id AC9FE800465; Fri, 31 Jul 2020 00:28:48 +0000 (UTC) Received: from T590 (ovpn-12-75.pek2.redhat.com [10.72.12.75]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 6B2C11992D; Fri, 31 Jul 2020 00:28:42 +0000 (UTC) Date: Fri, 31 Jul 2020 08:28:38 +0800 From: Ming Lei To: Sagi Grimberg Cc: Jens Axboe , Christoph Hellwig , linux-block@vger.kernel.org, Lai Jiangshan , "Paul E . McKenney" , Josh Triplett , Bart Van Assche Subject: Re: [RFC PATCH] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING Message-ID: <20200731002838.GB1717993@T590> References: <20200728134938.1505467-1-ming.lei@redhat.com> <20200729102856.GA1563056@T590> <20200729154957.GD1698748@T590> <20200730145325.GA1710335@T590> <57689a6d-9e6f-bb28-dd5f-f575988e7cb6@grimberg.me> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <57689a6d-9e6f-bb28-dd5f-f575988e7cb6@grimberg.me> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Thu, Jul 30, 2020 at 09:10:48AM -0700, Sagi Grimberg wrote: > > > > > > > > In case of BLK_MQ_F_BLOCKING, blk-mq uses SRCU to mark read critical > > > > > > > section during dispatching request, then request queue quiesce is based on > > > > > > > SRCU. What we want to get is low cost added in fast path. > > > > > > > > > > > > > > However, from srcu_read_lock/srcu_read_unlock implementation, not see > > > > > > > it is quicker than percpu refcount, so use percpu_ref to implement > > > > > > > queue quiesce. This usage is cleaner and simpler & enough for implementing > > > > > > > queue quiesce. The main requirement is to make sure all read sections to observe > > > > > > > QUEUE_FLAG_QUIESCED once blk_mq_quiesce_queue() returns. > > > > > > > > > > > > > > Also it becomes much easier to add interface of async queue quiesce. > > > > > > > > > > > > BTW, no obvious IOPS difference is observed with this patch applied when running > > > > > > io on null_blk(blocking, submit_queues=32) in one dual-socket, 32cores system. > > > > > > > > > > Thanks Ming, can you test for non-blocking on the same setup? > > > > > > > > OK, I can do that, but this patch supposes to not affect non-blocking, > > > > care to share your motivation for testing non-blocking? > > > > > > I think it will be a significant improvement to have a single code path. > > > The code will be more robust and we won't need to face issues that are > > > specific for blocking. > > > > > > If the cost is negligible, I think the upside is worth it. > > > > > > > rcu_read_lock and rcu_read_unlock has been proved as efficient enough, > > and I don't think percpu_refcount is better than it, so I'd suggest to > > not switch non-blocking into this way. > > It's not a matter of which is better, its a matter of making the code > more robust because it has a single code-path. If moving to percpu_ref > is negligible, I would suggest to move both, I don't want to have two > completely different mechanism for blocking vs. non-blocking. RCU and SRCU have been different mechanism already. > > > BTW, in case of blocking, one hctx may dispatch at most one request because there > > is only single .run_work, even though when .queue_rq() is slept, that said > > blk_mq_submit_bio() queues bio in sync style. This way won't be very efficient. > > So percpu_refcount should be good enough for blocking code path, but may not be > > well enough for non-blocking case. > > Not sure what you mean, the percpu_ref is taken exactly where rcu is > taken, not sure what is the difference. My point is that blocking can't be efficient enough, when .queue_rq() is slept, no any request can be queued to this hctx any more. thanks, Ming