The time spent in io_schedule() and also the interrupt latency are
significant when submitting direct I/O to a UFS device. Hence this patch
that implements polling support. User space software can enable polling by
passing the RWF_HIPRI flag to the preadv2() system call or the
IORING_SETUP_IOPOLL flag to the io_uring interface.

Although the block layer supports to partition the tag space for
interrupt-based completions (HCTX_TYPE_DEFAULT) purposes and polling
(HCTX_TYPE_POLL), the choice has been made to use the same hardware queue
for both hctx types because partitioning the tag space would negatively
affect performance.

On my test setup this patch increases IOPS from 2736 to 22000 (8x) for the
following test:

for hipri in 0 1; do
    fio --ioengine=io_uring --iodepth=1 --rw=randread \
    --runtime=60 --time_based=1 --direct=1 --name=qd1 \
    --filename=/dev/block/sda --ioscheduler=none --gtod_reduce=1 \
    --norandommap --hipri=$hipri
done

Link: https://lore.kernel.org/r/20211203231950.193369-18-bvanassche@acm.orgTested-by: Bean Huo <beanhuo@micron.com>
Reviewed-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>