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). >> >> 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. Cheers, Hannes -- Dr. Hannes Reinecke Kernel Storage Architect hare@xxxxxxx +49 911 74053 688 SUSE Software Solutions Germany GmbH, 90409 Nürnberg GF: F. Imendörffer, HRB 36809 (AG Nürnberg)
