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 A96FFC4332F for ; Wed, 19 Oct 2022 00:35:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229453AbiJSAfc (ORCPT ); Tue, 18 Oct 2022 20:35:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48418 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229437AbiJSAfb (ORCPT ); Tue, 18 Oct 2022 20:35:31 -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 ESMTPS id A36FD87FA5 for ; Tue, 18 Oct 2022 17:35:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1666139729; 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=7cKhGZbNn0cTfINoGM+8H9su2ksUdFx27vnXufiDesc=; b=NPdWplPMaAAiKOZuz37yQJmnHzdr6FlwXmpRHVR3/LazTpmcqojmHDUSXDE6ws9Hy/Hms4 GaVHZvtIzWFGDvnxIm4v7aR61Em0eS0MGjgb8d9zeQSmvwxARbDhNKPfdqVd5XYuUKJSKi eEHa1qQyqU1/dWMzX9RKJIjhffPoF3w= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-49-pGDy-2YKOKuCvWFRkmTPJg-1; Tue, 18 Oct 2022 20:35:25 -0400 X-MC-Unique: pGDy-2YKOKuCvWFRkmTPJg-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 488DB862FDF; Wed, 19 Oct 2022 00:35:25 +0000 (UTC) Received: from T590 (ovpn-8-20.pek2.redhat.com [10.72.8.20]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 6B05140E0420; Wed, 19 Oct 2022 00:35:19 +0000 (UTC) Date: Wed, 19 Oct 2022 08:35:16 +0800 From: Ming Lei To: Christoph Hellwig Cc: "Paul E. McKenney" , Chao Leng , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, sagi@grimberg.me, kbusch@kernel.org, axboe@kernel.dk Subject: Re: [PATCH v2 1/2] blk-mq: add tagset quiesce interface Message-ID: References: <20221013094450.5947-1-lengchao@huawei.com> <20221013094450.5947-2-lengchao@huawei.com> <20221017133906.GA24492@lst.de> <20221017152136.GI5600@paulmck-ThinkPad-P17-Gen-1> <20221017153105.GA32509@lst.de> <20221017224115.GJ5600@paulmck-ThinkPad-P17-Gen-1> <20221018051956.GA18802@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221018051956.GA18802@lst.de> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Tue, Oct 18, 2022 at 07:19:56AM +0200, Christoph Hellwig wrote: > On Mon, Oct 17, 2022 at 03:41:15PM -0700, Paul E. McKenney wrote: > > Then the big question is "how long do the SRCU readers run?" > > > > If all of the readers ran for exactly the same duration, there would be > > little point in having more than one srcu_struct. > > The SRCU readers are the I/O dispatch, which will have quite similar > runtimes for the different queues. > > > If the kernel knew up front how long the SRCU readers for a given entry > > would run, it could provide an srcu_struct structure for each duration. > > For a (fanciful) example, you could have one srcu_struct structure for > > SSDs, another for rotating rust, a third for LAN-attached storage, and > > a fourth for WAN-attached storage. Maybe a fifth for lunar-based storage. > > All the different request_queues in a tag_set are for the same device. > There might be some corner cases like storare arrays where they have > different latencies. But we're not even waiting for the I/O completion > here, this just protects the submission. > > > Does that help, or am I off in the weeds here? > > I think this was very helpful, and at least to be moving the srcu_struct > to the tag_set sounds like a good idea to explore. > > Ming, anything I might have missed? I think it is fine to move it to tag_set, this way could simplify a lot. The effect could be that blk_mq_quiesce_queue() becomes a little slow, but it is always in slow path, and synchronize_srcu() won't wait new read-side critical-section. Just one corner case, in case of BLK_MQ_F_BLOCKING, is there any such driver which may block too long in ->queue_rq() sometime? such as wait for dozens of seconds. thanks, Ming