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Message-Id: <0AC85BFB-F887-4512-887D-A2E7D36D9C52@linux.dev> Date: Tue, 27 Aug 2024 15:31:27 +0800 From: Muchun Song <muchun.song@...ux.dev> To: Yu Kuai <yukuai1@...weicloud.com> Cc: Muchun Song <songmuchun@...edance.com>, Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>, "open list:BLOCK LAYER" <linux-block@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, "yukuai (C)" <yukuai3@...wei.com> Subject: Re: [PATCH 2/4] block: fix ordering between checking BLK_MQ_S_STOPPED and adding requests to hctx->dispatch > On Aug 26, 2024, at 16:53, Yu Kuai <yukuai1@...weicloud.com> wrote: > > Hi, > > 在 2024/08/26 16:35, Muchun Song 写道: >>> On Aug 22, 2024, at 11:54, Yu Kuai <yukuai1@...weicloud.com> wrote: >>> >>> Hi, >>> >>> 在 2024/08/19 11:49, Muchun Song 写道: >>>> On Mon, Aug 19, 2024 at 10:28 AM Ming Lei <ming.lei@...hat.com> wrote: >>>>> >>>>> Hi Muchun, >>>>> >>>>> On Sun, Aug 11, 2024 at 06:19:19PM +0800, Muchun Song wrote: >>>>>> Supposing the following scenario with a virtio_blk driver. >>>>>> >>>>>> CPU0 CPU1 >>>>>> >>>>>> blk_mq_try_issue_directly() >>>>>> __blk_mq_issue_directly() >>>>>> q->mq_ops->queue_rq() >>>>>> virtio_queue_rq() >>>>>> blk_mq_stop_hw_queue() >>>>>> virtblk_done() >>>>>> blk_mq_request_bypass_insert() blk_mq_start_stopped_hw_queues() >>>>>> /* Add IO request to dispatch list */ 1) store blk_mq_start_stopped_hw_queue() >>>>>> clear_bit(BLK_MQ_S_STOPPED) 3) store >>>>>> blk_mq_run_hw_queue() blk_mq_run_hw_queue() >>>>>> if (!blk_mq_hctx_has_pending()) if (!blk_mq_hctx_has_pending()) 4) load >>>>>> return return >>>>>> blk_mq_sched_dispatch_requests() blk_mq_sched_dispatch_requests() >>>>>> if (blk_mq_hctx_stopped()) 2) load if (blk_mq_hctx_stopped()) >>>>>> return return >>>>>> __blk_mq_sched_dispatch_requests() __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 BLK_MQ_S_STOPPED 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. >>>>> >>>>> Yeah, it is one kind of race which is triggered when adding request into >>>>> ->dispatch list after returning STS_RESOURCE. We were troubled by lots of >>>>> such kind of race. >>>> Yes. I saw the similar fix for BLK_MQ_S_SCHED_RESTART. >>>>> >>>>> stopping queue is used in very less drivers, and its only purpose should >>>>> be for throttling hw queue in case that low level queue is busy. There seems >>>>> more uses of blk_mq_stop_hw_queues(), but most of them should be replaced >>>>> with blk_mq_quiesce_queue(). >>>>> >>>>> IMO, fixing this kind of issue via memory barrier is too tricky to >>>>> maintain cause WRITE/READ dependency is very hard to follow. I'd suggest to >>>>> make memory barrier solution as the last resort, and we can try to figure >>>>> out other easier & more reliable way first. >>>> I do agree it is hard to maintain the dependencies in the future. We should >>>> propose an easy-maintainable solution. But I thought it is a long-term issue >>>> throughout different stable linux distros. Adding a mb is the easy way to fix >>>> the problem (the code footprint is really small), so it will be very >>>> easy for others >>>> to backport those bug fixes to different stable linux distros. Therefore, mb >>>> should be an interim solution. Then, we could improve it based on the solution >>>> you've proposed below. What do you think? >>> >>> I'll agree with Ming, let's figure out a better fix first. Easy to backport to stables is not first consideration. >> Hi Kuai, >> All right. I usually focus on MM, it seems there is a gap between MM and BLock. >> Anyway, let's figure out if there is any good solution. >>>> Thanks, >>>> Muchun. >>>>> >>>>> One idea I thought of is to call blk_mq_request_bypass_insert()(or rename >>>>> & export it) before calling blk_mq_stop_hw_queue() in driver, then >>>>> return new status code STS_STOP_DISPATCH for notifying blk-mq to stop >>>>> dispatch simply. >>> >>> New status code look good to me, however, I wonder can we just remove >>> the problematic blk_mq_stop_hw_queue(), and replace it by handling the >>> new status from block layer? >>> >>> - Passing the new status to blk_mq_run_dispatch_ops, and quiesce with >> I didn't fully understand your suggestion. Let me ask some questions. >> blk_mq_stop_hw_queue() is usually called in blk_mq_ops->queue_rq path, >> it'll be easy for this case to pass the new status to blk_mq_run_dispatch_ops. >> Should we remove blk_mq_stop_hw_queues() as well? How to pass the new >> status to blk_mq_run_dispatch_ops in this case? > > For queue_rq from dispatch path, it can be removed. However, it is > called from remove path as well, I don't check yet if it can be removed > there, that's another story. The reason why I asked this question is that blk_mq_stop_hw_queues() also needs to be fixed. See my patch 3. > > And just add a return value for dispatch_ops to pass status. > > Thanks, > Kuai > >>> the new status, if no request is inflight, unquiesce immediately; >> Actually, I didn't understand how to avoid the above race. May you elaborate >> the scenario? Sorry for repeating, I didn't get your point here. May you elaborate your suggestion? Thanks very much. >> Muhcun, >> Thanks. >>> - unquiesce is any IO is done afterwards; >> .
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