Re: REQ_HIPRI and SCSI mid-level

[Hannes Reinecke <hare@suse.de>]

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@suse.de			               +49 911 74053 688
SUSE Software Solutions Germany GmbH, 90409 Nürnberg
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