block: flush plug based on hardware and software queue order

block: flush plug based on hardware and software queue order

commit 26fed4ac4eab09c27fbae1859696cc38f0536407 upstream.

We used to sort the plug list if we had multiple queues before dispatching
requests to the IO scheduler. This usually isn't needed, but for certain
workloads that interleave requests to disks, it's a less efficient to
process the plug list one-by-one if everything is interleaved.

Don't sort the list, but skip through it and flush out entries that have
the same target at the same time.

Fixes: df87eb0fce8f ("block: get rid of plug list sorting")
Reported-and-tested-by: Song Liu <song@kernel.org>
Reviewed-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>