Re: REQ_HIPRI and SCSI mid-level

[Ming Lei <ming.lei@redhat.com>]

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