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[75.85.219.51]) by smtp.gmail.com with ESMTPSA id d18sm2473267pjv.34.2020.05.20.08.23.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 20 May 2020 08:23:02 -0700 (PDT) Subject: Re: io_uring vs CPU hotplug, was Re: [PATCH 5/9] blk-mq: don't set data->ctx and data->hctx in blk_mq_alloc_request_hctx From: Jens Axboe To: Christoph Hellwig , Ming Lei Cc: linux-kernel@vger.kernel.org, Thomas Gleixner , linux-block@vger.kernel.org, John Garry , Bart Van Assche , Hannes Reinecke , io-uring@vger.kernel.org References: <20200518093155.GB35380@T590> <87imgty15d.fsf@nanos.tec.linutronix.de> <20200518115454.GA46364@T590> <20200518131634.GA645@lst.de> <20200518141107.GA50374@T590> <20200518165619.GA17465@lst.de> <20200519015420.GA70957@T590> <20200519153000.GB22286@lst.de> <20200520011823.GA415158@T590> <20200520030424.GI416136@T590> <20200520080357.GA4197@lst.de> <8f893bb8-66a9-d311-ebd8-d5ccd8302a0d@kernel.dk> Message-ID: <448d3660-0d83-889b-001f-a09ea53fa117@kernel.dk> Date: Wed, 20 May 2020 09:20:50 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <8f893bb8-66a9-d311-ebd8-d5ccd8302a0d@kernel.dk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 5/20/20 8:45 AM, Jens Axboe wrote: > On 5/20/20 2:03 AM, Christoph Hellwig wrote: >> On Wed, May 20, 2020 at 11:04:24AM +0800, Ming Lei wrote: >>> On Wed, May 20, 2020 at 09:18:23AM +0800, Ming Lei wrote: >>>> On Tue, May 19, 2020 at 05:30:00PM +0200, Christoph Hellwig wrote: >>>>> On Tue, May 19, 2020 at 09:54:20AM +0800, Ming Lei wrote: >>>>>> As Thomas clarified, workqueue hasn't such issue any more, and only other >>>>>> per CPU kthreads can run until the CPU clears the online bit. >>>>>> >>>>>> So the question is if IO can be submitted from such kernel context? >>>>> >>>>> What other per-CPU kthreads even exist? >>>> >>>> I don't know, so expose to wider audiences. >>> >>> One user is io uring with IORING_SETUP_SQPOLL & IORING_SETUP_SQ_AFF, see >>> io_sq_offload_start(), and it is a IO submission kthread. >> >> As far as I can tell that code is buggy, as it still needs to migrate >> the thread away when the cpu is offlined. This isn't a per-cpu kthread >> in the sene of having one for each CPU. >> >> Jens? > > It just uses kthread_create_on_cpu(), nothing home grown. Pretty sure > they just break affinity if that CPU goes offline. Just checked, and it works fine for me. If I create an SQPOLL ring with SQ_AFF set and bound to CPU 3, if CPU 3 goes offline, then the kthread just appears unbound but runs just fine. When CPU 3 comes online again, the mask appears correct. So don't think there's anything wrong on that side. The affinity is a performance optimization, not a correctness issue. Really not much we can do if the chosen CPU is offlined, apart from continue to chug along. -- Jens Axboe