On Tue, Jun 01, 2021 at 02:19:10PM +0200, Hannes Reinecke wrote: > On 5/28/21 3:32 AM, Ming Lei wrote: > > On Thu, May 27, 2021 at 05:43:07PM +0200, Hannes Reinecke wrote: > >> On 5/25/21 6:03 PM, Douglas Gilbert wrote: > >>> On 2021-05-21 5:56 p.m., Douglas Gilbert wrote: > >>>> The REQ_HIPRI flag on requests is associated with blk_poll() (aka iopoll) > >>>> and assumes the user space (or some higher level) will be calling > >>>> blk_poll() on requests marked with REQ_HIPRI and that will lead to their > >>>> completion. > >>>> > >>>> In lk 5.13-rc1 the megaraid and scsi_debug LLDs support blk_poll() [seen > >>>> by searching for 'mq_poll'] with more to follow, I assume. I have tested > >>>> blk_poll() on the scsi_debug driver using both fio and the new sg driver. > >>>> It works well with one caveat: as long as there isn't an error. > >>>> After fighting with that error processing from the ULD side (i.e. the > >>>> new sg driver) and the LLD side I am concluding that the glue that > >>>> holds them together, that is, the mid-level is not as REQ_HIPRI aware > >>>> as it should be. > >>>> > >>>> Yes REQ_HIPRI is there in scsi_lib.c but it is missing from scsi_error.c > >>>> How can scsi_error.c re-issue requests _without_ taking into account > >>>> that the original was issued with REQ_HIPRI ? Well I don't know but I'm > >>>> pretty sure that is close to the area that I see causing problems > >>>> (mainly lockups). > >>>> > >>>> As an example the scsi_debug driver has an in-use bitmap that when a new > >>>> request arrives the code looks for an empty slot. Due to (incorrect) > >>>> parameter setup that may fail. If the driver returns: > >>>> device_qfull_result = (DID_OK << 16) | SAM_STAT_TASK_SET_FULL; > >>>> then I see lock-ups if the request in question has REQ_HIPRI set. > >>>> > >>>> If that is changed to: > >>>> device_qfull_result = (DID_ABORT << 16) | SAM_STAT_TASK_SET_FULL; > >>>> then my user space test program sees that error and aborts showing the > >>>> TASK SET FULL SCSI status. That is much better than a lockup ... > >>>> > >> That's because with the first result the command is requeued (due to > >> DID_OK), whereas in the latter result the command is aborted (due to > >> DID_ABORT). > >> > >> So the question really is whether we should retry the commands which have > >> REQ_HIPRI set, or whether we shouldn't rather complete them with appropriate > >> error code. > >> A bit like enhanced BLOCK_PC requests, if you will. > >> > >>>> Having played around with variants of the above for a few weeks, I'd > >>>> like to throw this problem into the open :-) > >>>> > >>>> > >>>> Suggestion: perhaps the eh could give up immediately on any request > >>>> with REQ_HIPRI set (i.e. make it a higher level layer's problem). > >> > >> Like I said above: it's not only scsi EH which would need to be modified, > >> but possibly also the result evaluation in scsi_decide_disposition(); it's > >> questionable whether a HIPRI command should be requeued at all. > > > > Why can't HIPRI req be requeued? > > > Oh, it can. > As I said: it's questionable; HIPRI / polled completions are just that, > polling for completions. And a completion indicating a requeue is > _still_ a completion. > So one could argue that we should return here (as it's a completion, and > we're polling for completion). No, blk_poll() actually poll for bio's completion instead of request's completion. If the submitted HIPRI bio isn't completed, the polling won't be stopped. > > >> > >> But this might even affect the NVMe folks; they do return commands with > >> BLK_STS_RESOURCE, too. > > > > Block layer will be responsible for re-queueing BLK_STS_RESOURCE requests, > > so still not understand why it is one issue for HIPRI req. Also > > rq->mq_hctx won't be changed since its allocation, blk_poll() > > will keep polling on the correct hw queue for reaping the IO. > > > As mentioned above, I was talking about completions indicating a requeue. > Requeues due to resource shortage on the initiator side would of course > be requeued. blk_poll() doesn't care request requeue, what matters is that if the HIPRI bio is completed or not. So either timeout or EH codes needn't to handle HIPRI IO specially. Thanks, Ming
