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 ... 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).
Hmmm, no response. Any LLD that decides to support blk_poll() and relies on the mid-level eh handling will get burnt by this. And those LLDs that do their own eh handling needs to take care not to replicate the mid-level's eh handling in this case. An even simpler approach for the mid-level eh handling to address (at least partially) this problem, is to do: rqq->cmd_flags &= ~REQ_HIPRI; on retried requests. Clearing the REQ_HIPRI flag, if present, should make the LLD behave the way the mid-level eh processing expects. Doug Gilbert
