[JFFS2] force the jffs2 GC daemon to behave a bit better

I've noticed some pretty poor behavior on OLPC machines after bootup, when
gdm/X are starting.  The GCD monopolizes the scheduler (which in turns
means it gets to do more nand i/o), which results in processes taking much
much longer than they should to start.

As an example, on an OLPC machine going from OFW to a usable X (via
auto-login gdm) takes 2m 30s.  The majority of this time is consumed by
the switch into graphical mode.  With this patch, we cut a full 60s off of
bootup time.  After bootup, things are much snappier as well.

Note that we have seen a CRC node error with this patch that causes the machine
to fail to boot, but we've also seen that problem without this patch.

Signed-off-by: Andres Salomon <dilinger@debian.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

fs/jffs2/background.c

@@ -95,13 +95,17 @@ static int jffs2_garbage_collect_thread(void *_c)
 			spin_unlock(&c->erase_completion_lock);
 			
 
-		/* 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();
+		/* Problem - immediately after bootup, the GCD spends a lot
+		 * of time in places like jffs2_kill_fragtree(); so much so
+		 * that userspace processes (like gdm and X) are starved
+		 * despite plenty of cond_resched()s and renicing.  Yield()
+		 * doesn't help, either (presumably because userspace and GCD
+		 * are generally competing for a higher latency resource -
+		 * disk).
+		 * This forces the GCD to slow the hell down.   Pulling an
+		 * inode in with read_inode() is much preferable to having
+		 * the GC thread get there first. */
+		schedule_timeout_interruptible(msecs_to_jiffies(50));
 
 		/* Put_super will send a SIGKILL and then wait on the sem.
 		 */