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From: Muchun Song In-Reply-To: <3D6BB557-E9D1-4421-A541-CA2BF742506C@linux.dev> Date: Thu, 12 Sep 2024 14:27:25 +0800 Cc: Muchun Song , Yu Kuai , "open list:BLOCK LAYER" , LKML , stable@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: <92F2578A-EA56-4904-8E96-DD2BE3B0F875@linux.dev> References: <20240903081653.65613-1-songmuchun@bytedance.com> <20240903081653.65613-3-songmuchun@bytedance.com> <91ce06c7-6965-4d1d-8ed4-d0a6f01acecf@kernel.dk> <3D6BB557-E9D1-4421-A541-CA2BF742506C@linux.dev> To: Ming Lei , Jens Axboe X-Migadu-Flow: FLOW_OUT > On Sep 12, 2024, at 11:27, Muchun Song wrote: >=20 >=20 >=20 >> On Sep 11, 2024, at 11:54, Ming Lei wrote: >>=20 >> 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. >>>>=20 >>>> CPU0 CPU1 >>>>=20 >>>> 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 >>>>=20 >>>> 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. >>>>=20 >>>> 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. >>>=20 >>> Same comment here, 72-74 chars wide please. >>>=20 >>>> 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); >>>>=20 >>>> +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 =3D !blk_queue_quiesced(hctx->queue) && >>>> + blk_mq_hctx_has_pending(hctx)); >>>> + return need_run; >>>> +} >>>=20 >>> This __blk_mq_run_dispatch_ops() is also way too wide, why didn't = you >>> just break it like where you copied it from? >>>=20 >>>> + >>>> /** >>>> * 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) >>>>=20 >>>> might_sleep_if(!async && hctx->flags & BLK_MQ_F_BLOCKING); >>>>=20 >>>> - /* >>>> - * 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 =3D !blk_queue_quiesced(hctx->queue) && >>>> - blk_mq_hctx_has_pending(hctx)); >>>> + need_run =3D blk_mq_hw_queue_need_run(hctx); >>>> + if (!need_run) { >>>> + unsigned long flags; >>>>=20 >>>> - 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 =3D blk_mq_hw_queue_need_run(hctx); >>>> + spin_unlock_irqrestore(&hctx->queue->queue_lock, = flags); >>>> + >>>> + if (!need_run) >>>> + return; >>>> + } >>>=20 >>> 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. >>=20 >> 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(). >=20 > Hi Ming and Jens, >=20 > I use call_srcu/call_rcu to make it non-sleepable. Does this make = sense to you? Sorry for the noise. call_srcu/call_rcu can't be easy to do this. Because call_srcu/call_rcu could be issued twice if users try to unquiesce the queue again before the callback of blk_mq_run_hw_queues_rcu has been executed. Thanks. >=20 > diff --git a/block/blk-mq.c b/block/blk-mq.c > index 12bf38bec1044..86cdff28b2ce6 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -247,6 +247,13 @@ void blk_mq_quiesce_queue(struct request_queue = *q) > } > EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue); >=20 > +static void blk_mq_run_hw_queues_rcu(struct rcu_head *rh) > +{ > + struct request_queue *q =3D container_of(rh, struct = request_queue, > + rcu_head); > + blk_mq_run_hw_queues(q, true); > +} > + > /* > * blk_mq_unquiesce_queue() - counterpart of blk_mq_quiesce_queue() > * @q: request queue. > @@ -269,8 +276,13 @@ void blk_mq_unquiesce_queue(struct request_queue = *q) > spin_unlock_irqrestore(&q->queue_lock, flags); >=20 > /* dispatch requests which are inserted during quiescing */ > - if (run_queue) > - blk_mq_run_hw_queues(q, true); > + if (run_queue) { > + if (q->tag_set->flags & BLK_MQ_F_BLOCKING) > + call_srcu(q->tag_set->srcu, &q->rcu_head, > + blk_mq_run_hw_queues_rcu); > + else > + call_rcu(&q->rcu_head, = blk_mq_run_hw_queues_rcu); > + } > } > EXPORT_SYMBOL_GPL(blk_mq_unquiesce_queue); >=20 >>=20 >>=20 >> Thanks, >> Ming