[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>

diff --git a/fs/jffs2/background.c b/fs/jffs2/background.c
index 3cceef4..e958010 100644 (file)
--- a/fs/jffs2/background.c
+++ b/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.
                 */