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charset=us-ascii Content-Disposition: inline In-Reply-To: <91ce06c7-6965-4d1d-8ed4-d0a6f01acecf@kernel.dk> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 On Tue, Sep 10, 2024 at 07:22:16AM -0600, Jens Axboe wrote: > On 9/3/24 2:16 AM, Muchun Song wrote: > > Supposing the following scenario. > > > > 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 > > > > 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. > > > > So the first solution is to 1) add a pair of memory barrier to fix the > > problem, another solution is to 2) use hctx->queue->queue_lock to synchronize > > QUEUE_FLAG_QUIESCED. Here, we chose 2) to fix it since memory barrier is not > > easy to be maintained. > > Same comment here, 72-74 chars wide please. > > > diff --git a/block/blk-mq.c b/block/blk-mq.c > > index b2d0f22de0c7f..ac39f2a346a52 100644 > > --- a/block/blk-mq.c > > +++ b/block/blk-mq.c > > @@ -2202,6 +2202,24 @@ 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) > > +{ > > + bool need_run; > > + > > + /* > > + * When queue is quiesced, we may be switching io scheduler, or > > + * updating nr_hw_queues, or other things, and we can't run queue > > + * any more, even blk_mq_hctx_has_pending() can't be called safely. > > + * > > + * And queue will be rerun in blk_mq_unquiesce_queue() if it is > > + * quiesced. > > + */ > > + __blk_mq_run_dispatch_ops(hctx->queue, false, > > + need_run = !blk_queue_quiesced(hctx->queue) && > > + blk_mq_hctx_has_pending(hctx)); > > + return need_run; > > +} > > This __blk_mq_run_dispatch_ops() is also way too wide, why didn't you > just break it like where you copied it from? > > > + > > /** > > * blk_mq_run_hw_queue - Start to run a hardware queue. > > * @hctx: Pointer to the hardware queue to run. > > @@ -2222,20 +2240,23 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async) > > > > might_sleep_if(!async && hctx->flags & BLK_MQ_F_BLOCKING); > > > > - /* > > - * When queue is quiesced, we may be switching io scheduler, or > > - * updating nr_hw_queues, or other things, and we can't run queue > > - * any more, even __blk_mq_hctx_has_pending() can't be called safely. > > - * > > - * And queue will be rerun in blk_mq_unquiesce_queue() if it is > > - * 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) { > > + unsigned long flags; > > > > - if (!need_run) > > - return; > > + /* > > + * synchronize with blk_mq_unquiesce_queue(), becuase we check > > + * if hw queue is quiesced locklessly above, we need the use > > + * ->queue_lock to make sure we see the up-to-date status to > > + * not miss rerunning the hw queue. > > + */ > > + 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; > > + } > > Is this not solvable on the unquiesce side instead? It's rather a shame > to add overhead to the fast path to avoid a race with something that's > super unlikely, like quisce. Yeah, it can be solved by adding synchronize_rcu()/srcu() in unquiesce side, but SCSI may call it in non-sleepable context via scsi_internal_device_unblock_nowait(). Thanks, Ming