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=ham 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 B6943C282D7 for ; Wed, 30 Jan 2019 10:38:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 88C6020811 for ; Wed, 30 Jan 2019 10:38:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730699AbfA3Kit (ORCPT ); Wed, 30 Jan 2019 05:38:49 -0500 Received: from szxga07-in.huawei.com ([45.249.212.35]:39984 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726548AbfA3Kit (ORCPT ); Wed, 30 Jan 2019 05:38:49 -0500 Received: from DGGEMS406-HUB.china.huawei.com (unknown [172.30.72.60]) by Forcepoint Email with ESMTP id 5688DAD717B61B5EF2AF; Wed, 30 Jan 2019 18:38:47 +0800 (CST) Received: from [127.0.0.1] (10.202.226.43) by DGGEMS406-HUB.china.huawei.com (10.3.19.206) with Microsoft SMTP Server id 14.3.408.0; Wed, 30 Jan 2019 18:38:38 +0800 Subject: Re: Question on handling managed IRQs when hotplugging CPUs To: Keith Busch References: <20190129154433.GF15302@localhost.localdomain> <757902fc-a9ea-090b-7853-89944a0ce1b5@huawei.com> <20190129172059.GC17132@localhost.localdomain> CC: "tglx@linutronix.de" , Christoph Hellwig , Marc Zyngier , "axboe@kernel.dk" , Peter Zijlstra , Michael Ellerman , Linuxarm , "linux-kernel@vger.kernel.org" , Hannes Reinecke From: John Garry Message-ID: <3fe63dab-0791-f476-69c4-9866b70e8520@huawei.com> Date: Wed, 30 Jan 2019 10:38:18 +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: <20190129172059.GC17132@localhost.localdomain> 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 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. Thanks, John > > . >