On Fri, May 21, 2021 at 05:56:19PM -0400, 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 ... > > 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). One invariant is that the polling will be kept as running until the associated iocb/bio is completed. So I understand it should be fine for timeout handler /EH to ignore REQ_HIPRI. That said the associated iocb/bio will be reaped by upper layer if EH/timeout handler makes progress. Or can you explain the scsi-debug REQ_HIPRI lockup in a bit detail? Thanks, Ming
