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 E7F20FA373E for ; Tue, 25 Oct 2022 07:40:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231892AbiJYHkr (ORCPT ); Tue, 25 Oct 2022 03:40:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59038 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231769AbiJYHko (ORCPT ); Tue, 25 Oct 2022 03:40:44 -0400 Received: from frasgout.his.huawei.com (frasgout.his.huawei.com [185.176.79.56]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DCFD2114DCF; Tue, 25 Oct 2022 00:40:42 -0700 (PDT) Received: from frapeml500006.china.huawei.com (unknown [172.18.147.226]) by frasgout.his.huawei.com (SkyGuard) with ESMTP id 4MxP2G6sHsz6H7M1; Tue, 25 Oct 2022 15:38:46 +0800 (CST) Received: from lhrpeml500003.china.huawei.com (7.191.162.67) by frapeml500006.china.huawei.com (7.182.85.219) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Tue, 25 Oct 2022 09:40:39 +0200 Received: from [10.48.157.17] (10.48.157.17) by lhrpeml500003.china.huawei.com (7.191.162.67) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Tue, 25 Oct 2022 08:40:38 +0100 Message-ID: <3513b14c-14e0-b865-628e-a83521090de9@huawei.com> Date: Tue, 25 Oct 2022 08:40:38 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.6.1 From: John Garry Subject: Re: [PATCH] blk-mq: Properly init bios from blk_mq_alloc_request_hctx() To: Ming Lei CC: , , , , Bart Van Assche References: <1666454846-11749-1-git-send-email-john.garry@huawei.com> <99c6ca81-746d-85f4-04d3-49d7a3de611b@huawei.com> <360c78dc-65ce-362f-389d-075f2259ce5b@huawei.com> In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.48.157.17] X-ClientProxiedBy: lhrpeml500005.china.huawei.com (7.191.163.240) To lhrpeml500003.china.huawei.com (7.191.162.67) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 25/10/2022 01:34, Ming Lei wrote: >>>> but sometimes we just need to allocate for a specific HW >>>> queue... >>>> >>>> For my usecase of interest, it should not impact if the cpumask of the HW >>>> queue goes offline after selecting the cpu in blk_mq_alloc_request_hctx(), >>>> so any race is ok ... I think. >>>> >>>> However it should be still possible to make blk_mq_alloc_request_hctx() more >>>> robust. How about using something like work_on_cpu_safe() to allocate and >>>> execute the request with blk_mq_alloc_request() on a cpu associated with the >>>> HW queue, such that we know the cpu is online and stays online until we >>>> execute it? Or also extent to work_on_cpumask_safe() variant, so that we >>>> don't need to try all cpus in the mask (to see if online)? >>> But all cpus on this hctx->cpumask could become offline. >> If all hctx->cpumask are offline then we should not allocate a request and >> this is acceptable. Maybe I am missing your point. > As you saw, this API has the above problem too, but any one of CPUs > may become online later, maybe just during blk_mq_alloc_request_hctx(), > and it is easy to cause inconsistence. > > You didn't share your use case, but for nvme connection request, if it > is 1:1 mapping, if any one of CPU becomes offline, the controller > initialization could be failed, that isn't good from user viewpoint at > all. My use case is in SCSI EH domain. For my HBA controller of interest, to abort an erroneous IO we must send a controller proprietary abort command on same HW queue as original command. So we would need to allocate this abort request for a specific HW queue. I mentioned before that if no hctx->cpumask is online then we don't need to allocate a request. That is because if no hctx->cpumask is online, this means that original erroneous IO must be completed due to nature of how blk-mq cpu hotplug handler works, i.e. drained, and then we don't actually need to abort it any longer, so ok to not get a request. I have an RFC series for this work in which I am using blk_mq_alloc_request_hctx(). However, as I mentioned before, I can experiment with using something like work_on_cpu_safe() to alloc and execute the abort request to safeguard against cpu hotplug events. Thanks, John