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=-6.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 3141BC433E0 for ; Wed, 29 Jul 2020 04:37:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1563D20786 for ; Wed, 29 Jul 2020 04:37:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726336AbgG2Eh0 (ORCPT ); Wed, 29 Jul 2020 00:37:26 -0400 Received: from mail-pj1-f66.google.com ([209.85.216.66]:51570 "EHLO mail-pj1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726286AbgG2EhZ (ORCPT ); Wed, 29 Jul 2020 00:37:25 -0400 Received: by mail-pj1-f66.google.com with SMTP id c6so1293126pje.1 for ; Tue, 28 Jul 2020 21:37:25 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=XU31FClko5F8gicbnWPOCsqy42fRTkYxwgHqqzVb+Ho=; b=Ud3ZaiN0sDn+BAMNYyLdWniu8KF3tepTCtBIhAorWKm6tV0Hg80sxL9Q9GgTCOdRAR 3A3xxv/6FeEi8v7zlD0qyiXSsf3ORoGonzMX6rvC528foVHAmic56AO9T6+3kj4+CgIl mh1hEglLJTu18TFm7rERqDGXX/vX7d67TV5dRlfJHV+ineaCZ58nQ0VMMQNcCfr6T6gg 479XrNajFCK4OKbQzOobX+XjRhZU5epd/rQlei80rJGxs0xTnBRFa1JJsgpl9uz34D92 8hkXL6wVuz40p+NnhJTnEDaLyJqgdxzTet9mXa5wOcwPqamYb3UMDhRnRkfonu4nbuyS Cg0Q== X-Gm-Message-State: AOAM5329AU/DzM2H0BAvgRuoSys9emoz8bo/b3Rqn62w7gXCnOuW1tTv 9x4CPtVkeP1TKYDglqqMFPI= X-Google-Smtp-Source: ABdhPJzrJrVRsxMmyLmDqk439jheErXZ3UST5+jlVprC28TP0ny6Lfwa/+oo3ejMLNGcZL6m1d0O2g== X-Received: by 2002:a17:902:8697:: with SMTP id g23mr25366202plo.94.1595997445233; Tue, 28 Jul 2020 21:37:25 -0700 (PDT) Received: from ?IPv6:2601:647:4802:9070:fcc5:69d8:6e20:4fd1? ([2601:647:4802:9070:fcc5:69d8:6e20:4fd1]) by smtp.gmail.com with ESMTPSA id a193sm655256pfa.105.2020.07.28.21.37.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 28 Jul 2020 21:37:24 -0700 (PDT) Subject: Re: [PATCH v5 1/2] blk-mq: add tagset quiesce interface To: paulmck@kernel.org Cc: Ming Lei , Christoph Hellwig , Jens Axboe , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, Chao Leng , Keith Busch , Ming Lin References: <20200727231022.307602-1-sagi@grimberg.me> <20200727231022.307602-2-sagi@grimberg.me> <20200728071859.GA21629@lst.de> <20200728091633.GB1326626@T590> <20200728135436.GP9247@paulmck-ThinkPad-P72> <20200729003124.GT9247@paulmck-ThinkPad-P72> <07c90cf1-bb6f-a343-b0bf-4c91b9acb431@grimberg.me> <20200729041004.GV9247@paulmck-ThinkPad-P72> From: Sagi Grimberg Message-ID: <57f76f9c-6fb9-b6f1-ba85-1594755e60f3@grimberg.me> Date: Tue, 28 Jul 2020 21:37:23 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200729041004.GV9247@paulmck-ThinkPad-P72> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org >>>> 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... > > OK, I will bite... Do multiple lower-order allocations (page size is > still lower-order, correct?) and link them together. > > Sorry, couldn't resist... Possible, but I didn't want us to resort to all this complexity and thought we can find a better, simpler solution.