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? Thanks, Ming
