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 021AFC169C4 for ; Tue, 29 Jan 2019 17:13:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C7D88214DA for ; Tue, 29 Jan 2019 17:13:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729006AbfA2RNC (ORCPT ); Tue, 29 Jan 2019 12:13:02 -0500 Received: from szxga05-in.huawei.com ([45.249.212.191]:3237 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728379AbfA2RNC (ORCPT ); Tue, 29 Jan 2019 12:13:02 -0500 Received: from DGGEMS406-HUB.china.huawei.com (unknown [172.30.72.58]) by Forcepoint Email with ESMTP id 3951922AE9A7DD864459; Wed, 30 Jan 2019 01:12:58 +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 01:12:47 +0800 From: John Garry Subject: Re: Question on handling managed IRQs when hotplugging CPUs To: Keith Busch References: <20190129154433.GF15302@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 Message-ID: <757902fc-a9ea-090b-7853-89944a0ce1b5@huawei.com> Date: Tue, 29 Jan 2019 17:12:40 +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: <20190129154433.GF15302@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 15:44, Keith Busch wrote: > On Tue, Jan 29, 2019 at 03:25:48AM -0800, John Garry wrote: >> Hi, >> >> I have a question on $subject which I hope you can shed some light on. >> >> According to commit c5cb83bb337c25 ("genirq/cpuhotplug: Handle managed >> IRQs on CPU hotplug"), if we offline the last CPU in a managed IRQ >> affinity mask, the IRQ is shutdown. >> >> The reasoning is that this IRQ is thought to be associated with a >> specific queue on a MQ device, and the CPUs in the IRQ affinity mask are >> the same CPUs associated with the queue. So, if no CPU is using the >> queue, then no need for the IRQ. >> >> However how does this handle scenario of last CPU in IRQ affinity mask >> being offlined while IO associated with queue is still in flight? >> >> Or if we make the decision to use queue associated with the current CPU, >> and then that CPU (being the last CPU online in the queue's IRQ >> afffinity mask) goes offline and we finish the delivery with another CPU? >> >> In these cases, when the IO completes, it would not be serviced and timeout. >> >> I have actually tried this on my arm64 system and I see IO timeouts. > > 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. Thanks, John > . >