On Tue, Aug 27, 2024 at 4:17 PM Muchun Song <muchun.song@xxxxxxxxx> wrote: > > > > > 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. Yes, as I mentioned, the race only exists on two slow code paths, we seldom use barrier in slow paths, in which traditional lock can provide a simpler & more readable solution. Anytime, READ/WRITE dependency implied in any barrier is hard to follow. Thanks,