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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 61C82C3F2D1 for ; Wed, 4 Mar 2020 09:39:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3FFB72073D for ; Wed, 4 Mar 2020 09:39:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729023AbgCDJjV (ORCPT ); Wed, 4 Mar 2020 04:39:21 -0500 Received: from lhrrgout.huawei.com ([185.176.76.210]:2507 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728953AbgCDJjU (ORCPT ); Wed, 4 Mar 2020 04:39:20 -0500 Received: from lhreml701-cah.china.huawei.com (unknown [172.18.7.108]) by Forcepoint Email with ESMTP id 1DA68866E4B2B3F597A6; Wed, 4 Mar 2020 09:39:19 +0000 (GMT) Received: from lhreml724-chm.china.huawei.com (10.201.108.75) by lhreml701-cah.china.huawei.com (10.201.108.42) with Microsoft SMTP Server (TLS) id 14.3.408.0; Wed, 4 Mar 2020 09:39:18 +0000 Received: from [127.0.0.1] (10.202.226.45) by lhreml724-chm.china.huawei.com (10.201.108.75) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.1713.5; Wed, 4 Mar 2020 09:39:15 +0000 Subject: Re: [PATCH v2 10/11] megaraid_sas: Use Block layer API to check SCSI device in-flight IO requests To: Sumit Saxena CC: Hannes Reinecke , Anand Lodnoor , Linux SCSI List , Kashyap Desai , Kiran Kumar Kasturi , Sankar Patra , Sasikumar PC , Shivasharan Srikanteshwara , Chandrakanth Patil , Ming Lei , "Bart Van Assche" , "Martin K . Petersen" References: <1579000882-20246-1-git-send-email-anand.lodnoor@broadcom.com> <1579000882-20246-11-git-send-email-anand.lodnoor@broadcom.com> <7ca1562c-7a7a-17c5-2429-9725d465a4a8@suse.de> <93deab34-53a3-afcf-4862-6b168a9f60cc@huawei.com> <79fe7843-9d71-bdde-957c-32dde22437d9@suse.de> <5ac6fd4f-eef8-700b-89d2-c8b3cd6e12ca@huawei.com> From: John Garry Message-ID: <46c35a02-2223-dd1b-2acc-6deeab478886@huawei.com> Date: Wed, 4 Mar 2020 09:39:13 +0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.1.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.202.226.45] X-ClientProxiedBy: lhreml725-chm.china.huawei.com (10.201.108.76) To lhreml724-chm.china.huawei.com (10.201.108.75) X-CFilter-Loop: Reflected Sender: linux-scsi-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-scsi@vger.kernel.org >> >> OK, I have a rough idea of the concept. And again I'd say megaraid sas >> may not be a good candidate to expose > 1 HW queues, as we hide HW >> queues and don't maintain the symmetry with blk-mq layer. > Sorry, my last response was not very clear. I was referring to reply > queues as HW queues > not submission queues. I agree with you, since megaraid_sas HW has > single submission > queue so >1 HW queue would not help to improve performance. Testing > done by us on shared > tagset patches worked by you/Hannes was to ensure no performance drop > from single HW > submission queue based driver. OK, but I still have concern with this. That's your choice. >> >> Indeed, I do not even expect a performance increase in exposing > 1 HW >> queues since the driver already uses the reply map + managed interrupts. >> >> The main reason for that change in some drivers - apart from losing the >> duplicated ugliness of the reply map - is to leverage the blk-mq feature >> to drain a hctx for CPU hotplug [0] - is this something which megaraid >> sas is vulnerable to and would benefit from? > "megaraid_sas" driver would be benefited with draining of IO > completions directed to > hotplugged(offlined) CPU. With current driver IO completion would > hang, if CPU on which IO is to be > completed goes offline. But that feature will only work for the queues which you expose. For the low-latency queues, there would be no draining*. However, the low-latency interrupts are not managed; as such, I think that their interrupts would migrate when their cpumask goes offline, rather than being shutdown, so not vulnerable to this problem. * In principle, since you can submit the scsi request on different hw queue than expected from blk-mq perspective, when we offline the cpu which blk-mq set to submit on, blk-mq may actually wait for requests to complete on these low-latency queues and addition to the HW queue which blk-mq thought that the request would be submitted on - again, not ideal, and may cause problems. Thanks, John