libaio,io_uring: relax cmdprio_percentage constraints

libaio,io_uring: relax cmdprio_percentage constraints

In fio, a job IO priority is controlled with the prioclass and prio
options and these options cannot be used together with the
cmdprio_percentage option.

Allow a user to have async IO priorities default to the job defined
IO priority by removing the mutual exclusion between the options
cmdprio_percentage and prioclass/prio.

With the introduction of the cmdprio_class option, an async IO priority
may be lower than the job default priority, resulting in reversed clat
statistics showed for high and low priority IOs when fio completes.
Solve this by setting an io_u IO_U_F_PRIORITY flag depending on a
comparison between the async IO priority and job default IO priority.

When an async IO is issued without a priority set, Linux kernel will
execute it using the IO priority of the issuing context set with
ioprio_set(). This works fine for libaio, where the context will be
the same as the context that submitted the IO.

However, io_uring can be used with a kernel thread that performs
block device IO submissions (sqthread_poll). Therefore, for io_uring,
an IO sqe ioprio field must be set to the job default priority unless
the IO priority is set according to the job cmdprio_percentage value.

Because of this, IO uring already did set sqe->ioprio even when only
prio/prioclass was used. See commit b7ed2a862dda ("io_uring: set sqe
iopriority, if prio/prioclass is set"). In order to make the code easier
to maintain, handle all I/O priority preparations in the same function.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>