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([2605:e000:100e:8c61:f02d:fa41:676:76e9]) by smtp.gmail.com with ESMTPSA id 19sm4972461pjl.52.2020.05.21.11.45.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 21 May 2020 11:45:13 -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 To: Thomas Gleixner , Ming Lei Cc: Christoph Hellwig , linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, John Garry , Bart Van Assche , Hannes Reinecke , io-uring@vger.kernel.org, Peter Zijlstra References: <20200520011823.GA415158@T590> <20200520030424.GI416136@T590> <20200520080357.GA4197@lst.de> <8f893bb8-66a9-d311-ebd8-d5ccd8302a0d@kernel.dk> <448d3660-0d83-889b-001f-a09ea53fa117@kernel.dk> <87tv0av1gu.fsf@nanos.tec.linutronix.de> <2a12a7aa-c339-1e51-de0d-9bc6ced14c64@kernel.dk> <87eereuudh.fsf@nanos.tec.linutronix.de> <20200521022746.GA730422@T590> <87367tvh6g.fsf@nanos.tec.linutronix.de> <20200521092340.GA751297@T590> <87pnaxt9nv.fsf@nanos.tec.linutronix.de> From: Jens Axboe Message-ID: <15f9f975-1baf-dc90-5730-00df08829523@kernel.dk> Date: Thu, 21 May 2020 12:45:11 -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: <87pnaxt9nv.fsf@nanos.tec.linutronix.de> 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 5/21/20 12:39 PM, Thomas Gleixner wrote: > Ming, > > Ming Lei writes: >> On Thu, May 21, 2020 at 10:13:59AM +0200, Thomas Gleixner wrote: >>> Ming Lei writes: >>>> On Thu, May 21, 2020 at 12:14:18AM +0200, Thomas Gleixner wrote: >>>> - otherwise, the kthread just retries and retries to allocate & release, >>>> and sooner or later, its time slice is consumed, and migrated out, and the >>>> cpu hotplug handler will get chance to run and move on, then the cpu is >>>> shutdown. >>> >>> 1) This is based on the assumption that the kthread is in the SCHED_OTHER >>> scheduling class. Is that really a valid assumption? >> >> Given it is unlikely path, we can add msleep() before retrying when INACTIVE bit >> is observed by current thread, and this way can avoid spinning and should work >> for other schedulers. > > That should work, but pretty is something else > >>> >>> 2) What happens in the following scenario: >>> >>> unplug >>> >>> mq_offline >>> set_ctx_inactive() >>> drain_io() >>> >>> io_kthread() >>> try_queue() >>> wait_on_ctx() >>> >>> Can this happen and if so what will wake up that thread? >> >> drain_io() releases all tag of this hctx, then wait_on_ctx() will be waken up >> after any tag is released. > > drain_io() is already done ... > > So looking at that thread function: > > static int io_sq_thread(void *data) > { > struct io_ring_ctx *ctx = data; > > while (...) { > .... > to_submit = io_sqring_entries(ctx); > > --> preemption > > hotplug runs > mq_offline() > set_ctx_inactive(); > drain_io(); > finished(); > > --> thread runs again > > mutex_lock(&ctx->uring_lock); > ret = io_submit_sqes(ctx, to_submit, NULL, -1, true); > mutex_unlock(&ctx->uring_lock); > > .... > > if (!to_submit || ret == -EBUSY) > ... > wait_on_ctx(); > > Can this happen or did drain_io() already take care of the 'to_submit' > items and the call to io_submit_sqes() turns into a zero action ? > > If the above happens then nothing will wake it up because the context > draining is done and finished. Again, this is mixing up io_uring and blk-mq. Maybe it's the fact that both use 'ctx' that makes this confusing. On the blk-mq side, the 'ctx' is the per-cpu queue context, for io_uring it's the io_uring instance. io_sq_thread() doesn't care about any sort of percpu mappings, it's happy as long as it'll keep running regardless of whether or not the optional pinned CPU is selected and then offlined. -- Jens Axboe