xen/blkback: Move the plugging/unplugging to a higher level.

We used to the plug/unplug on the submit_bio. But that means
if within a stream of WRITE, WRITE, WRITE,...,WRITE we have
one READ, it could stall the pipeline (as the 'submio_bio'
could trigger the unplug_fnc to be called and stall/sync
when doing the READ). Instead we want to move the unplugging
when the whole (or as a much as possible) ring buffer has been
processed. This also eliminates us doing plug/unplug for
each request.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkback/blkback.c

@@ -276,6 +276,8 @@ int xen_blkif_schedule(void *arg)
 		printk(KERN_DEBUG "%s: started\n", current->comm);
 
 	while (!kthread_should_stop()) {
+		struct blk_plug plug;
+
 		if (try_to_freeze())
 			continue;
 		if (unlikely(vbd->size != vbd_sz(vbd)))
@@ -292,9 +294,13 @@ int xen_blkif_schedule(void *arg)
 		blkif->waiting_reqs = 0;
 		smp_mb(); /* clear flag *before* checking for work */
 
+		blk_start_plug(&plug);
+
 		if (do_block_io_op(blkif))
 			blkif->waiting_reqs = 1;
 
+		blk_finish_plug(&plug);
+
 		if (log_stats && time_after(jiffies, blkif->st_print))
 			print_stats(blkif);
 	}
@@ -547,7 +553,6 @@ static void dispatch_rw_block_io(struct blkif_st *blkif,
 	struct bio *biolist[BLKIF_MAX_SEGMENTS_PER_REQUEST];
 	int i, nbio = 0;
 	int operation;
-	struct blk_plug plug;
 
 	switch (req->operation) {
 	case BLKIF_OP_READ:
@@ -660,15 +665,9 @@ static void dispatch_rw_block_io(struct blkif_st *blkif,
 	 */
 	atomic_set(&pending_req->pendcnt, nbio);
 
-	/* Get a reference count for the disk queue and start sending I/O */
-	blk_start_plug(&plug);
-
 	for (i = 0; i < nbio; i++)
 		submit_bio(operation, biolist[i]);
 
-	blk_finish_plug(&plug);
-	/* Let the I/Os go.. */
-
 	if (operation == READ)
 		blkif->st_rd_sect += preq.nr_sects;
 	else if (operation == WRITE || operation == WRITE_BARRIER)