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 CB807C433E0 for ; Fri, 7 Aug 2020 09:25:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A6F55221E5 for ; Fri, 7 Aug 2020 09:25:22 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="PN8z7gRD" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727042AbgHGJZU (ORCPT ); Fri, 7 Aug 2020 05:25:20 -0400 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:54552 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727030AbgHGJZU (ORCPT ); Fri, 7 Aug 2020 05:25:20 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1596792317; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=5zNghwZyDwNMJFv9scQbFvX4Q5eB7i0ZQE3vtTwERyw=; b=PN8z7gRDuqdOiIjzSoe+nG8IgWJUo5PFsylc0B+hYVFhJSo7/B/q90w/GE/TTXTekx/Z4I xmpzLUTSlk3RMQGdmKYsHEtswrYhLLLwvcJf60VND+y/hkc9DeVyvkulP4m0FuBUbzA/Ox 7YBK55iM2IqMtwq96Ypb7Yd71oLS1CQ= 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-207-6DgKfVeOMj2xYj1wbWQ_LA-1; Fri, 07 Aug 2020 05:25:13 -0400 X-MC-Unique: 6DgKfVeOMj2xYj1wbWQ_LA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 858C379EC0; Fri, 7 Aug 2020 09:25:11 +0000 (UTC) Received: from T590 (ovpn-13-53.pek2.redhat.com [10.72.13.53]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 193A887A6D; Fri, 7 Aug 2020 09:25:01 +0000 (UTC) Date: Fri, 7 Aug 2020 17:24:57 +0800 From: Ming Lei To: Chao Leng Cc: Sagi Grimberg , Keith Busch , paulmck@kernel.org, Christoph Hellwig , Jens Axboe , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, Ming Lin Subject: Re: [PATCH v5 1/2] blk-mq: add tagset quiesce interface Message-ID: <20200807092457.GA2112310@T590> References: <20200728071859.GA21629@lst.de> <20200728091633.GB1326626@T590> <20200728135436.GP9247@paulmck-ThinkPad-P72> <20200729003124.GT9247@paulmck-ThinkPad-P72> <07c90cf1-bb6f-a343-b0bf-4c91b9acb431@grimberg.me> <20200729005942.GA2729664@dhcp-10-100-145-180.wdl.wdc.com> <2f17c8ed-99f6-c71c-edd1-fd96481f432c@grimberg.me> <31a9ba72-1322-4b7c-fb73-db0cb52989da@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <31a9ba72-1322-4b7c-fb73-db0cb52989da@huawei.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Fri, Aug 07, 2020 at 05:04:38PM +0800, Chao Leng wrote: > > > On 2020/7/29 12:39, Sagi Grimberg wrote: > > > > > > > > Dynamically allocating each one is possible but not very scalable. > > > > > > > > > > > > The question is if there is some way, we can do this with on-stack > > > > > > or a single on-heap rcu_head or equivalent that can achieve the same > > > > > > effect. > > > > > > > > > > If the hctx structures are guaranteed to stay put, you could count > > > > > them and then do a single allocation of an array of rcu_head structures > > > > > (or some larger structure containing an rcu_head structure, if needed). > > > > > You could then sequence through this array, consuming one rcu_head per > > > > > hctx as you processed it.  Once all the callbacks had been invoked, > > > > > it would be safe to free the array. > > > > > > > > > > Sounds too simple, though.  So what am I missing? > > > > > > > > We don't want higher-order allocations... > > > > > > So: > > > > > >    (1) We don't want to embed the struct in the hctx because we allocate > > >    so many of them that this is non-negligable to add for something we > > >    typically never use. > > > > > >    (2) We don't want to allocate dynamically because it's potentially > > >    huge. > > > > > > As long as we're using srcu for blocking hctx's, I think it's "pick your > > > poison". > > > > > > Alternatively, Ming's percpu_ref patch(*) may be worth a look. > > > > > >   * https://www.spinics.net/lists/linux-block/msg56976.html1 > > I'm not opposed to having this. Will require some more testing > > as this affects pretty much every driver out there.. > > > > If we are going with a lightweight percpu_ref, can we just do > > it also for non-blocking hctx and have a single code-path? > > . > I tried to optimize the patch，support for non blocking queue and > blocking queue. > See next email. Please see the following thread: https://lore.kernel.org/linux-block/05f75e89-b6f7-de49-eb9f-a08aa4e0ba4f@kernel.dk/ Both Keith and Jens didn't think it is a good idea. Thanks, Ming