aio: only use blk plugs for > 2 depth submissions

Plugging is meant to optimize submission of a string of IOs, if we don't
have more than 2 being submitted, don't bother setting up a plug.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/aio.c

@@ -70,6 +70,12 @@ struct aio_ring {
 	struct io_event		io_events[0];
 }; /* 128 bytes + ring size */
 
+/*
+ * Plugging is meant to work with larger batches of IOs. If we don't
+ * have more than the below, then don't bother setting up a plug.
+ */
+#define AIO_PLUG_THRESHOLD	2
+
 #define AIO_RING_PAGES	8
 
 struct kioctx_table {
@@ -1921,7 +1927,8 @@ SYSCALL_DEFINE3(io_submit, aio_context_t, ctx_id, long, nr,
 	if (nr > ctx->nr_events)
 		nr = ctx->nr_events;
 
-	blk_start_plug(&plug);
+	if (nr > AIO_PLUG_THRESHOLD)
+		blk_start_plug(&plug);
 	for (i = 0; i < nr; i++) {
 		struct iocb __user *user_iocb;
 
@@ -1934,7 +1941,8 @@ SYSCALL_DEFINE3(io_submit, aio_context_t, ctx_id, long, nr,
 		if (ret)
 			break;
 	}
-	blk_finish_plug(&plug);
+	if (nr > AIO_PLUG_THRESHOLD)
+		blk_finish_plug(&plug);
 
 	percpu_ref_put(&ctx->users);
 	return i ? i : ret;
@@ -1961,7 +1969,8 @@ COMPAT_SYSCALL_DEFINE3(io_submit, compat_aio_context_t, ctx_id,
 	if (nr > ctx->nr_events)
 		nr = ctx->nr_events;
 
-	blk_start_plug(&plug);
+	if (nr > AIO_PLUG_THRESHOLD)
+		blk_start_plug(&plug);
 	for (i = 0; i < nr; i++) {
 		compat_uptr_t user_iocb;
 
@@ -1974,7 +1983,8 @@ COMPAT_SYSCALL_DEFINE3(io_submit, compat_aio_context_t, ctx_id,
 		if (ret)
 			break;
 	}
-	blk_finish_plug(&plug);
+	if (nr > AIO_PLUG_THRESHOLD)
+		blk_finish_plug(&plug);
 
 	percpu_ref_put(&ctx->users);
 	return i ? i : ret;