cfq-iosched: enable full blkcg hierarchy support

With the previous two patches, all cfqg scheduling decisions are based
on vfraction and ready for hierarchy support.  The only thing which
keeps the behavior flat is cfqg_flat_parent() which makes vfraction
calculation consider all non-root cfqgs children of the root cfqg.

Replace it with cfqg_parent() which returns the real parent.  This
enables full blkcg hierarchy support for cfq-iosched.  For example,
consider the following hierarchy.

        root
      /      \
   A:500      B:250
  /     \
 AA:500  AB:1000

For simplicity, let's say all the leaf nodes have active tasks and are
on service tree.  For each leaf node, vfraction would be

 AA: (500  / 1500) * (500 / 750) =~ 0.2222
 AB: (1000 / 1500) * (500 / 750) =~ 0.4444
  B:                 (250 / 750) =~ 0.3333

and vdisktime will be distributed accordingly.  For more detail,
please refer to Documentation/block/cfq-iosched.txt.

v2: cfq-iosched.txt updated to describe group scheduling as suggested
    by Vivek.

v3: blkio-controller.txt updated.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>

1 files changed, 6 insertions, 15 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index ee34282..e8f3106 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -606,20 +606,11 @@ static inline struct cfq_group *blkg_to_cfqg(struct blkcg_gq *blkg)
 	return pd_to_cfqg(blkg_to_pd(blkg, &blkcg_policy_cfq));
 }
 
-/*
- * Determine the parent cfqg for weight calculation.  Currently, cfqg
- * scheduling is flat and the root is the parent of everyone else.
- */
-static inline struct cfq_group *cfqg_flat_parent(struct cfq_group *cfqg)
+static inline struct cfq_group *cfqg_parent(struct cfq_group *cfqg)
 {
-	struct blkcg_gq *blkg = cfqg_to_blkg(cfqg);
-	struct cfq_group *root;
-
-	while (blkg->parent)
-		blkg = blkg->parent;
-	root = blkg_to_cfqg(blkg);
+	struct blkcg_gq *pblkg = cfqg_to_blkg(cfqg)->parent;
 
-	return root != cfqg ? root : NULL;
+	return pblkg ? blkg_to_cfqg(pblkg) : NULL;
 }
 
 static inline void cfqg_get(struct cfq_group *cfqg)
@@ -722,7 +713,7 @@ static void cfq_pd_reset_stats(struct blkcg_gq *blkg)
 
 #else	/* CONFIG_CFQ_GROUP_IOSCHED */
 
-static inline struct cfq_group *cfqg_flat_parent(struct cfq_group *cfqg) { return NULL; }
+static inline struct cfq_group *cfqg_parent(struct cfq_group *cfqg) { return NULL; }
 static inline void cfqg_get(struct cfq_group *cfqg) { }
 static inline void cfqg_put(struct cfq_group *cfqg) { }
 
@@ -1290,7 +1281,7 @@ cfq_group_service_tree_add(struct cfq_rb_root *st, struct cfq_group *cfqg)
 	 * stops once an already activated node is met.  vfraction
 	 * calculation should always continue to the root.
 	 */
-	while ((parent = cfqg_flat_parent(pos))) {
+	while ((parent = cfqg_parent(pos))) {
 		if (propagate) {
 			propagate = !parent->nr_active++;
 			parent->children_weight += pos->weight;
@@ -1341,7 +1332,7 @@ cfq_group_service_tree_del(struct cfq_rb_root *st, struct cfq_group *cfqg)
 	pos->children_weight -= pos->leaf_weight;
 
 	while (propagate) {
-		struct cfq_group *parent = cfqg_flat_parent(pos);
+		struct cfq_group *parent = cfqg_parent(pos);
 
 		/* @pos has 0 nr_active at this point */
 		WARN_ON_ONCE(pos->children_weight);