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,URIBL_BLOCKED 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 666F4C169C4 for ; Tue, 29 Jan 2019 11:54:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 40EE62083B for ; Tue, 29 Jan 2019 11:54:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731946AbfA2Lys (ORCPT ); Tue, 29 Jan 2019 06:54:48 -0500 Received: from smtp.nue.novell.com ([195.135.221.5]:41316 "EHLO smtp.nue.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730486AbfA2Lyr (ORCPT ); Tue, 29 Jan 2019 06:54:47 -0500 Received: from [10.160.4.48] (charybdis.suse.de [149.44.162.66]) by smtp.nue.novell.com with ESMTP (TLS encrypted); Tue, 29 Jan 2019 12:54:45 +0100 Subject: Re: Question on handling managed IRQs when hotplugging CPUs To: John Garry , tglx@linutronix.de, Christoph Hellwig Cc: Marc Zyngier , "axboe@kernel.dk" , Keith Busch , Peter Zijlstra , Michael Ellerman , Linuxarm , "linux-kernel@vger.kernel.org" , SCSI Mailing List References: From: Hannes Reinecke Message-ID: <5bff8227-16fd-6bca-c16e-3992ef6bec5a@suse.com> Date: Tue, 29 Jan 2019 12:54:44 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 1/29/19 12:25 PM, 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. > That actually is a very good question, and I have been wondering about this for quite some time. I find it a bit hard to envision a scenario where the IRQ affinity is automatically (and, more importantly, atomically!) re-routed to one of the other CPUs. And even it it were, chances are that there are checks in the driver _preventing_ them from handling those requests, seeing that they should have been handled by another CPU ... I guess the safest bet is to implement a 'cleanup' worker queue which is responsible of looking through all the outstanding commands (on all hardware queues), and then complete those for which no corresponding CPU / irqhandler can be found. But I defer to the higher authorities here; maybe I'm totally wrong and it's already been taken care of. But if there is no generic mechanism this really is a fit topic for LSF/MM, as most other drivers would be affected, too. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@suse.com +49 911 74053 688 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: F. Imendörffer, J. Smithard, D. Upmanyu, G. Norton HRB 21284 (AG Nürnberg)