> On Aug 27, 2024, at 15:24, Muchun Song <muchun.song@xxxxxxxxx> wrote: > > > >> On Aug 26, 2024, at 17:20, Ming Lei <ming.lei@xxxxxxxxxx> wrote: >> >> On Mon, Aug 26, 2024 at 03:33:18PM +0800, Muchun Song wrote: >>> >>> >>>> On Aug 26, 2024, at 15:06, Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote: >>>> >>>> On Fri, Aug 23, 2024 at 7:28 PM Ming Lei <ming.lei@xxxxxxxxxx> 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: >> > > If we prefer barrier-less approach, I think the following solution > will work as well, I'll use it in v2. Thanks. > >> >> 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); After some time thought, I think here we need a big comment to explain why we need to sync. Because there are other caller of blk_queue_quiesced() which do not need to hold ->queue_lock to sync. Then, I am thinking is ->queue_lock really easier to be maintained than mb? For developers, we still need to care about this, right? I don't see any obvious benefit. And the mb approach seems more efficient than spinlock. Something like: if (!need_run) { /* Add a comment here to explain what's going on here. */ smp_mb(); need_run = blk_mq_hw_queue_need_run(hctx); if (!need_run) return; } I am not objecting to your approach, I want to know if you insist on barrier-less approach here. If yes, I'm fine with this approach. I can use it in v2. Muhcun, Thanks. >> + need_run = blk_mq_hw_queue_need_run(hctx); > > One question here: should we use __blk_mq_run_dispatch_ops()? I saw a comment above. > It seems it is safe to call blk_mq_hw_queue_need_run under [s]rcu lock. > > Thanks. > >> + 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 > >