[JFFS2] Use yield() between GC passes in background thread.

The garbage collection thread is strictly an optimisation. Everything it
does would also be done just-in-time in the context of something in
userspace trying to access the file system.

Sometimes, however, it's a pessimisation. Especially during early boot
when it's checksumming nodes and scanning inodes which are shortly going
to be pulled in by read_inode anyway. We end up building the rbtree of
node coverage twice for the same inode.

By switching to yield() instead of cond_resched() in the main loop, we
observe boot times on the OLPC system going down from about 100 seconds to
60.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

fs/jffs2/background.c

@@ -99,7 +99,13 @@ static int jffs2_garbage_collect_thread(void *_c)
 		if (try_to_freeze())
 			continue;
 
-		cond_resched();
+		/* This thread is purely an optimisation. But if it runs when
+		   other things could be running, it actually makes things a
+		   lot worse. Use yield() and put it at the back of the runqueue
+		   every time. Especially during boot, pulling an inode in
+		   with read_inode() is much preferable to having the GC thread
+		   get there first. */
+		yield();
 
 		/* Put_super will send a SIGKILL and then wait on the sem.
 		 */