aio: only account I/O wait time in read_events if there are active requests

On 2.6.24, top started showing 100% iowait on one CPU when a UML instance was
running (but completely idle).  The UML code sits in io_getevents waiting for
an event to be submitted and completed.

Fix this by checking ctx->reqs_active before scheduling to determine whether
or not we are waiting for I/O.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Cc: Zach Brown <zach.brown@oracle.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 1 deletions

diff --git a/fs/aio.c b/fs/aio.c
index f12db41..9dec7d2 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1161,7 +1161,12 @@ retry:
 			ret = 0;
 			if (to.timed_out)	/* Only check after read evt */
 				break;
-			io_schedule();
+			/* Try to only show up in io wait if there are ops
+			 *  in flight */
+			if (ctx->reqs_active)
+				io_schedule();
+			else
+				schedule();
 			if (signal_pending(tsk)) {
 				ret = -EINTR;
 				break;