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Mon, 26 Aug 2024 05:20:55 -0400 X-MC-Unique: KZ5NH_XSNcO0E27HjFksIw-1 Received: from mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.40]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E63A31955D53; Mon, 26 Aug 2024 09:20:53 +0000 (UTC) Received: from fedora (unknown [10.72.116.156]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 1C3A219560A3; Mon, 26 Aug 2024 09:20:48 +0000 (UTC) Date: Mon, 26 Aug 2024 17:20:42 +0800 From: Ming Lei To: Muchun Song Cc: Muchun Song , Jens Axboe , "open list:BLOCK LAYER" , LKML , ming.lei@redhat.com Subject: Re: [PATCH 4/4] block: fix fix ordering between checking QUEUE_FLAG_QUIESCED and adding requests to hctx->dispatch Message-ID: References: <20240811101921.4031-1-songmuchun@bytedance.com> <20240811101921.4031-5-songmuchun@bytedance.com> <45A22FCE-10FA-485C-8624-F1F22086B5E9@linux.dev> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <45A22FCE-10FA-485C-8624-F1F22086B5E9@linux.dev> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 On Mon, Aug 26, 2024 at 03:33:18PM +0800, Muchun Song wrote: > > > > On Aug 26, 2024, at 15:06, Muchun Song wrote: > > > > On Fri, Aug 23, 2024 at 7:28 PM Ming Lei wrote: > >> > >> On Sun, Aug 11, 2024 at 06:19:21 PM +0800, Muchun Song wrote: > >>> Supposing the following scenario. > >>> > >>> CPU0 CPU1 > >>> > >>> blk_mq_request_issue_directly() blk_mq_unquiesce_queue() > >>> if (blk_queue_quiesced()) blk_queue_flag_clear(QUEUE_FLAG_QUIESCED) 3) store > >>> blk_mq_insert_request() blk_mq_run_hw_queues() > >>> /* blk_mq_run_hw_queue() > >>> * Add request to dispatch list or set bitmap of if (!blk_mq_hctx_has_pending()) 4) load > >>> * software queue. 1) store return > >>> */ > >>> blk_mq_run_hw_queue() > >>> if (blk_queue_quiesced()) 2) load > >>> return > >>> blk_mq_sched_dispatch_requests() > >>> > >>> The full memory barrier should be inserted between 1) and 2), as well as > >>> between 3) and 4) to make sure that either CPU0 sees QUEUE_FLAG_QUIESCED is > >>> cleared or CPU1 sees dispatch list or setting of bitmap of software queue. > >>> Otherwise, either CPU will not re-run the hardware queue causing starvation. > >> > >> Memory barrier shouldn't serve as bug fix for two slow code paths. > >> > >> One simple fix is to add helper of blk_queue_quiesced_lock(), and > >> call the following check on CPU0: > >> > >> if (blk_queue_quiesced_lock()) > >> blk_mq_run_hw_queue(); > > > > This only fixes blk_mq_request_issue_directly(), I think anywhere that > > matching this > > pattern (inserting a request to dispatch list and then running the > > hardware queue) > > should be fixed. And I think there are many places which match this > > pattern (E.g. > > blk_mq_submit_bio()). The above graph should be adjusted to the following. > > > > CPU0 CPU1 > > > > blk_mq_insert_request() 1) store blk_mq_unquiesce_queue() > > blk_mq_run_hw_queue() > > blk_queue_flag_clear(QUEUE_FLAG_QUIESCED) 3) store > > if (blk_queue_quiesced()) 2) load blk_mq_run_hw_queues() > > return blk_mq_run_hw_queue() > > blk_mq_sched_dispatch_requests() if > > (!blk_mq_hctx_has_pending()) 4) load > > return > > Sorry. There is something wrong with my email client. Resend the graph. > > CPU0 CPU1 > > blk_mq_insert_request() 1) store blk_mq_unquiesce_queue() > blk_mq_run_hw_queue() blk_queue_flag_clear(QUEUE_FLAG_QUIESCED) 3) store > if (blk_queue_quiesced()) 2) load blk_mq_run_hw_queues() > return blk_mq_run_hw_queue() > blk_mq_sched_dispatch_requests() if (!blk_mq_hctx_has_pending()) 4) load > return OK. The issue shouldn't exist if blk_queue_quiesced() return false in blk_mq_run_hw_queue(), so it is still one race in two slow paths? I guess the barrier-less approach should work too, such as: diff --git a/block/blk-mq.c b/block/blk-mq.c index e3c3c0c21b55..632261982a77 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -2202,6 +2202,12 @@ void blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, unsigned long msecs) } EXPORT_SYMBOL(blk_mq_delay_run_hw_queue); +static inline bool blk_mq_hw_queue_need_run(struct blk_mq_hw_ctx *hctx) +{ + return !blk_queue_quiesced(hctx->queue) && + blk_mq_hctx_has_pending(hctx); +} + /** * blk_mq_run_hw_queue - Start to run a hardware queue. * @hctx: Pointer to the hardware queue to run. @@ -2231,11 +2237,19 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async) * quiesced. */ __blk_mq_run_dispatch_ops(hctx->queue, false, - need_run = !blk_queue_quiesced(hctx->queue) && - blk_mq_hctx_has_pending(hctx)); + need_run = blk_mq_hw_queue_need_run(hctx)); - if (!need_run) - return; + if (!need_run) { + unsigned long flags; + + /* sync with unquiesce */ + spin_lock_irqsave(&hctx->queue->queue_lock, flags); + need_run = blk_mq_hw_queue_need_run(hctx); + spin_unlock_irqrestore(&hctx->queue->queue_lock, flags); + + if (!need_run) + return; + } if (async || !cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask)) { blk_mq_delay_run_hw_queue(hctx, 0); thanks, Ming