[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>

diff --git a/fs/jffs2/background.c b/fs/jffs2/background.c
index 6eb3dae..888f236 100644 (file)
--- a/fs/jffs2/background.c
+++ b/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.
                 */