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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=unavailable 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 55E88C282D9 for ; Thu, 31 Jan 2019 17:48:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 35BC420B1F for ; Thu, 31 Jan 2019 17:48:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727621AbfAaRsU (ORCPT ); Thu, 31 Jan 2019 12:48:20 -0500 Received: from szxga08-in.huawei.com ([45.249.212.255]:51468 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725849AbfAaRsU (ORCPT ); Thu, 31 Jan 2019 12:48:20 -0500 Received: from DGGEMS414-HUB.china.huawei.com (unknown [172.30.72.54]) by Forcepoint Email with ESMTP id 60DD867F5522B6745ED4; Fri, 1 Feb 2019 01:48:17 +0800 (CST) Received: from [127.0.0.1] (10.202.226.43) by DGGEMS414-HUB.china.huawei.com (10.3.19.214) with Microsoft SMTP Server id 14.3.408.0; Fri, 1 Feb 2019 01:48:09 +0800 Subject: Re: Question on handling managed IRQs when hotplugging CPUs To: Thomas Gleixner References: <20190129154433.GF15302@localhost.localdomain> <757902fc-a9ea-090b-7853-89944a0ce1b5@huawei.com> <20190129172059.GC17132@localhost.localdomain> <3fe63dab-0791-f476-69c4-9866b70e8520@huawei.com> CC: Keith Busch , Christoph Hellwig , "Marc Zyngier" , "axboe@kernel.dk" , "Peter Zijlstra" , Michael Ellerman , Linuxarm , "linux-kernel@vger.kernel.org" , Hannes Reinecke , "linux-scsi@vger.kernel.org" , "linux-block@vger.kernel.org" From: John Garry Message-ID: <86d5028d-44ab-3696-f7fe-828d7655faa9@huawei.com> Date: Thu, 31 Jan 2019 17:48:02 +0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.202.226.43] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 30/01/2019 12:43, Thomas Gleixner wrote: > On Wed, 30 Jan 2019, John Garry wrote: >> On 29/01/2019 17:20, Keith Busch wrote: >>> On Tue, Jan 29, 2019 at 05:12:40PM +0000, John Garry wrote: >>>> On 29/01/2019 15:44, Keith Busch wrote: >>>>> >>>>> Hm, we used to freeze the queues with CPUHP_BLK_MQ_PREPARE callback, >>>>> which would reap all outstanding commands before the CPU and IRQ are >>>>> taken offline. That was removed with commit 4b855ad37194f ("blk-mq: >>>>> Create hctx for each present CPU"). It sounds like we should bring >>>>> something like that back, but make more fine grain to the per-cpu >>>>> context. >>>>> >>>> >>>> Seems reasonable. But we would need it to deal with drivers where they >>>> only >>>> expose a single queue to BLK MQ, but use many queues internally. I think >>>> megaraid sas does this, for example. >>>> >>>> I would also be slightly concerned with commands being issued from the >>>> driver unknown to blk mq, like SCSI TMF. >>> >>> I don't think either of those descriptions sound like good candidates >>> for using managed IRQ affinities. >> >> I wouldn't say that this behaviour is obvious to the developer. I can't see >> anything in Documentation/PCI/MSI-HOWTO.txt >> >> It also seems that this policy to rely on upper layer to flush+freeze queues >> would cause issues if managed IRQs are used by drivers in other subsystems. >> Networks controllers may have multiple queues and unsoliciated interrupts. > > It's doesn't matter which part is managing flush/freeze of queues as long > as something (either common subsystem code, upper layers or the driver > itself) does it. > > So for the megaraid SAS example the BLK MQ layer obviously can't do > anything because it only sees a single request queue. But the driver could, > if the the hardware supports it. tell the device to stop queueing > completions on the completion queue which is associated with a particular > CPU (or set of CPUs) during offline and then wait for the on flight stuff > to be finished. If the hardware does not allow that, then managed > interrupts can't work for it. > A rough audit of current SCSI drivers tells that these set PCI_IRQ_AFFINITY in some path but don't set Scsi_host.nr_hw_queues at all: aacraid, be2iscsi, csiostor, megaraid, mpt3sas I don't know specific driver details, like changing completion queue. Thanks, John > Thanks, > > tglx > > . >