[PATCH] add rb_next in block throttle

[Hillf Danton]

When selecting group for dispatching, the first entry of the service
tree is sequentially dequeued, then the first entry is recomputed by
calling rb_first().

When the first is removed from service tree, new first could also be
computed with rb_next(), since it could be faster than rb_first in
this special environment.

Signed-off-by: Hillf Danton <dhillf@xxxxxxxxx>
---

--- a/block/blk-throttle.c	2010-11-01 19:54:12.000000000 +0800
+++ b/block/blk-throttle.c	2010-11-26 21:27:40.000000000 +0800
@@ -251,7 +251,7 @@ static void rb_erase_init(struct rb_node
 static void throtl_rb_erase(struct rb_node *n, struct throtl_rb_root *root)
 {
 	if (root->left == n)
-		root->left = NULL;
+		root->left = rb_next(n);
 	rb_erase_init(n, &root->rb);
 	--root->count;
 }
--
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