[PATCH] sched: vfs: fix scheduling latencies in prune_dcache() and select_parent()

The attached patch fixes long scheduling latencies in select_parent() and
prune_dcache().  The prune_dcache() lock-break is easy, but for
select_parent() the only viable solution i found was to break out if
there's a resched necessary - the reordering is not necessary and the
dcache scanning/shrinking will later on do it anyway.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/dcache.c

@@ -156,7 +156,7 @@ void dput(struct dentry *dentry)
 		spin_unlock(&dcache_lock);
 		return;
 	}
-			
+
 	/*
 	 * AV: ->d_delete() is _NOT_ allowed to block now.
 	 */
@@ -392,6 +392,8 @@ static void prune_dcache(int count)
 		struct dentry *dentry;
 		struct list_head *tmp;
 
+		cond_resched_lock(&dcache_lock);
+
 		tmp = dentry_unused.prev;
 		if (tmp == &dentry_unused)
 			break;
@@ -548,6 +550,13 @@ int have_submounts(struct dentry *parent)
  * list for prune_dcache(). We descend to the next level
  * whenever the d_subdirs list is non-empty and continue
  * searching.
+ *
+ * It returns zero iff there are no unused children,
+ * otherwise  it returns the number of children moved to
+ * the end of the unused list. This may not be the total
+ * number of unused children, because select_parent can
+ * drop the lock and return early due to latency
+ * constraints.
  */
 static int select_parent(struct dentry * parent)
 {
@@ -577,6 +586,15 @@ static int select_parent(struct dentry * parent)
 			dentry_stat.nr_unused++;
 			found++;
 		}
+
+		/*
+		 * We can return to the caller if we have found some (this
+		 * ensures forward progress). We'll be coming back to find
+		 * the rest.
+		 */
+		if (found && need_resched())
+			goto out;
+
 		/*
 		 * Descend a level if the d_subdirs list is non-empty.
 		 */
@@ -601,6 +619,7 @@ this_parent->d_parent->d_name.name, this_parent->d_name.name, found);
 #endif
 		goto resume;
 	}
+out:
 	spin_unlock(&dcache_lock);
 	return found;
 }