cfq-iosched: Add comments on update timing of weight

Explain that weight has to be updated on activation.
This complements previous fix e15693ef18e1 ("cfq-iosched: Fix wrong
children_weight calculation").

Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index d7494637c5db..3f31cf9508e6 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1272,6 +1272,9 @@ __cfq_group_service_tree_add(struct cfq_rb_root *st, struct cfq_group *cfqg)
         rb_insert_color(&cfqg->rb_node, &st->rb);
 }
 
+/*
+ * This has to be called only on activation of cfqg
+ */
 static void
 cfq_update_group_weight(struct cfq_group *cfqg)
 {
@@ -1303,6 +1306,11 @@ cfq_group_service_tree_add(struct cfq_rb_root *st, struct cfq_group *cfqg)
         /* add to the service tree */
         BUG_ON(!RB_EMPTY_NODE(&cfqg->rb_node));
 
+        /*
+         * Update leaf_weight.  We cannot update weight at this point
+         * because cfqg might already have been activated and is
+         * contributing its current weight to the parent's child_weight.
+         */
         cfq_update_group_leaf_weight(cfqg);
         __cfq_group_service_tree_add(st, cfqg);