dm mpath: delay the retry of a request if the target responded as busy

Add DM_ENDIO_DELAY_REQUEUE to allow request-based multipath's
multipath_end_io() to instruct dm-rq.c:dm_done() to delay a requeue.
This is beneficial to do if BLK_STS_RESOURCE is returned from the target
(because target is busy).

Relative to blk-mq: kick the hw queues via blk_mq_requeue_work(),
indirectly from dm-rq.c:__dm_mq_kick_requeue_list(), after a delay.

For old .request_fn: use blk_delay_queue().

bio-based multipath doesn't have feature parity with request-based for
retryable error requeues; that is something that'll need fixing in the
future.

Suggested-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Bart Van Assche <bart.vanassche@wdc.com>
[as interpreted from Bart's "... patch looks fine to me."]

drivers/md/dm-mpath.c

@@ -1585,7 +1585,10 @@ static int multipath_end_io(struct dm_target *ti, struct request *clone,
 	if (error && !noretry_error(error)) {
 		struct multipath *m = ti->private;
 
-		r = DM_ENDIO_REQUEUE;
+		if (error == BLK_STS_RESOURCE)
+			r = DM_ENDIO_DELAY_REQUEUE;
+		else
+			r = DM_ENDIO_REQUEUE;
 
 		if (pgpath)
 			fail_path(pgpath);

drivers/md/dm-rq.c

@@ -315,6 +315,10 @@ static void dm_done(struct request *clone, blk_status_t error, bool mapped)
 		/* The target wants to requeue the I/O */
 		dm_requeue_original_request(tio, false);
 		break;
+	case DM_ENDIO_DELAY_REQUEUE:
+		/* The target wants to requeue the I/O after a delay */
+		dm_requeue_original_request(tio, true);
+		break;
 	default:
 		DMWARN("unimplemented target endio return value: %d", r);
 		BUG();

include/linux/device-mapper.h

@@ -550,6 +550,7 @@ do {									\
 #define DM_ENDIO_DONE		0
 #define DM_ENDIO_INCOMPLETE	1
 #define DM_ENDIO_REQUEUE	2
+#define DM_ENDIO_DELAY_REQUEUE	3
 
 /*
  * Definitions of return values from target map function.
@@ -557,7 +558,7 @@ do {									\
 #define DM_MAPIO_SUBMITTED	0
 #define DM_MAPIO_REMAPPED	1
 #define DM_MAPIO_REQUEUE	DM_ENDIO_REQUEUE
-#define DM_MAPIO_DELAY_REQUEUE	3
+#define DM_MAPIO_DELAY_REQUEUE	DM_ENDIO_DELAY_REQUEUE
 #define DM_MAPIO_KILL		4
 
 #define dm_sector_div64(x, y)( \