linux-imx

scsi: lpfc: Separate CQ processing for nvmet_fc upcalls

Currently the driver is notified of new command frame receipt by CQEs. As
part of the CQE processing, the driver upcalls the nvmet_fc transport to
deliver the command. nvmet_fc, as part of receiving the command builds out
a context for it, where one of the first steps is to allocate memory for
the io.

When running with tests that do large ios (1MB), it was found on some
systems, the total number of outstanding I/O's, at 1MB per, completely
consumed the system's memory. Thus additional ios were getting blocked in
the memory allocator.  Given that this blocked the lpfc thread processing
CQEs, there were lots of other commands that were received and which are
then held up, and given CQEs are serially processed, the aggregate delays
for an IO waiting behind the others became cummulative - enough so that the
initiator hit timeouts for the ios.

The basic fix is to avoid the direct upcall and instead schedule a work
item for each io as it is received. This allows the cq processing to
complete very quickly, and each io can then run or block on it's own.
However, this general solution hurts latency when there are few ios.  As
such, implemented the fix such that the driver watches how many CQEs it has
processed sequentially in one run. As long as the count is below a
threshold, the direct nvmet_fc upcall will be made. Only when the count is
exceeded will it revert to work scheduling.

Given that debug of this showed a surprisingly long delay in cq processing,
the io timer stats were updated to better reflect the processing of the
different points.

Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>