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From: Muchun Song In-Reply-To: Date: Tue, 27 Aug 2024 15:31:27 +0800 Cc: Muchun Song , Ming Lei , Jens Axboe , "open list:BLOCK LAYER" , LKML , "yukuai (C)" Content-Transfer-Encoding: quoted-printable Message-Id: <0AC85BFB-F887-4512-887D-A2E7D36D9C52@linux.dev> References: <20240811101921.4031-1-songmuchun@bytedance.com> <20240811101921.4031-3-songmuchun@bytedance.com> <786a8d94-884c-8a31-151d-fdc82e1a0a63@huaweicloud.com> <3AC15539-1B9B-4996-A150-8CAB214159E5@linux.dev> To: Yu Kuai X-Migadu-Flow: FLOW_OUT > On Aug 26, 2024, at 16:53, Yu Kuai wrote: >=20 > Hi, >=20 > =E5=9C=A8 2024/08/26 16:35, Muchun Song =E5=86=99=E9=81=93: >>> On Aug 22, 2024, at 11:54, Yu Kuai wrote: >>>=20 >>> Hi, >>>=20 >>> =E5=9C=A8 2024/08/19 11:49, Muchun Song =E5=86=99=E9=81=93: >>>> On Mon, Aug 19, 2024 at 10:28=E2=80=AFAM Ming Lei = wrote: >>>>>=20 >>>>> Hi Muchun, >>>>>=20 >>>>> On Sun, Aug 11, 2024 at 06:19:19PM +0800, Muchun Song wrote: >>>>>> Supposing the following scenario with a virtio_blk driver. >>>>>>=20 >>>>>> CPU0 = CPU1 >>>>>>=20 >>>>>> 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() >>>>>>=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 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. >>>>>=20 >>>>> 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. >>>>>=20 >>>>> 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(). >>>>>=20 >>>>> 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? >>>=20 >>> 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. >>>>>=20 >>>>> 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. >>>=20 >>> 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? >>>=20 >>> - 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? >=20 > 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. >=20 > And just add a return value for dispatch_ops to pass status. >=20 > Thanks, > Kuai >=20 >>> 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; >> .