On 2/19/20 5:45 PM, Ming Lei wrote: > On Wed, Feb 19, 2020 at 03:47:50PM -0800, dongli.zhang@xxxxxxxxxx wrote: >> >> >> On 2/19/20 2:10 PM, Ming Lei wrote: >>> On Wed, Feb 19, 2020 at 08:36:15AM -0800, Christoph Hellwig wrote: >>>> On Sat, Feb 15, 2020 at 11:21:40AM +0800, Ming Lei wrote: >>>>> For some reason, device may be in one situation which can't handle >>>>> FS request, so STS_RESOURCE is always returned and the FS request >>>>> will be added to hctx->dispatch. However passthrough request may >>>>> be required at that time for fixing the problem. If passthrough >>>>> request is added to scheduler queue, there isn't any chance for >>>>> blk-mq to dispatch it given we prioritize requests in hctx->dispatch. >>>>> Then the FS IO request may never be completed, and IO hang is caused. >>>>> >>>>> So passthrough request has to be added to hctx->dispatch directly. >>>>> >>>>> Fix this issue by inserting passthrough request into hctx->dispatch >>>>> directly. Then it becomes consistent with original legacy IO request >>>>> path, in which passthrough request is always added to q->queue_head. >>>> >>>> Do you have a description of an actual problem this fixes? Maybe even >>>> a reproducer for blktests? >>>> >>> >>> It is reported by one RH customer in the following test case: >>> >>> 1) Start IO on Emulex FC host >>> 2) Fail one controller, wait 5 minutes >>> 3) Bring controller back online >>> >>> When we trace the problem, it is found that FS request started in device_add_disk() >>> from scsi disk probe context stuck because scsi_queue_rq() always return >>> STS_BUSY via scsi_setup_fs_cmnd() -> alua_prep_fn(). >>> >>> The kernel ALUA state is TRANSITIONING at that time, so it is reasonable to see >>> BLK_TYPE_FS requests won't go anywhere because of the check in alua_prep_fn(). >>> >>> However, the passthrough request(TEST UNIT READY) is submitted from alua_rtpg_work >>> when the FS request can't be dispatched to LLD. And SCSI stack should >>> have been allowed to handle this passthrough rquest. But it can't reach SCSI stack >>> via .queue_rq() because blk-mq won't dispatch it until hctx->dispatch is >>> empty. >>> >>> The legacy IO request code always added passthrough request into head of q->queue_head >>> directly instead of scheduler queue or sw queue, so no such issue. >>> >>> So far not figured out one blktests test case, but the problem is real. >>> >>> BTW, I just found we need the extra following change: >>> >>> @@ -1301,7 +1301,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, >>> q->mq_ops->commit_rqs(hctx); >>> >>> spin_lock(&hctx->lock); >>> - list_splice_init(list, &hctx->dispatch); >>> + list_splice_tail_init(list, &hctx->dispatch); >>> spin_unlock(&hctx->lock); >>> >> >> Is it fine to add to tail as the requests on dispatch would be reordered? > > Wrt. FS request: > > Firstly we never guarantee that the request is dispatched in order. > > Secondly and more importantly, request can be added into hctx->dispatch > in any order. One usual case is that request is added to hctx->dispatch > concurrently when .queue_rq() fails. On the other side, in case of not > concurrent adding to hctx->dispatch, after one request is added to > hctx->dispatch, we always dispatch request from hctx->dispatch first, > instead of dequeuing request from scheduler queue and adding them to > hctx->dispatch again after .queue_rq() fails. > >> >> A, B, C and D are on the list. Suppose A is failed and the new list would become >> B, C D, A? > > Right, I don't see there is any issue in this way, do you see issues? Thank you very much for the explanation. I do not see issue if order guarantee in hctx->dispatch is not required. Dongli Zhang
