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.
But this might even affect the NVMe folks; they do return commands with
BLK_STS_RESOURCE, too.
Maybe we should open a larger discussion on the block list.
Cheers,
Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
hare@xxxxxxx +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer
