cfq-iosched: implement cfq_group->nr_active and ->children_weight

To prepare for blkcg hierarchy support, add cfqg->nr_active and
->children_weight.  cfqg->nr_active counts the number of active cfqgs
at the cfqg's level and ->children_weight is sum of weights of those
cfqgs.  The level covers itself (cfqg->leaf_weight) and immediate
children.

The two values are updated when a cfqg enters and leaves the group
service tree.  Unless the hierarchy is very deep, the added overhead
should be negligible.

Currently, the parent is determined using cfqg_flat_parent() which
makes the root cfqg the parent of all other cfqgs.  This is to make
the transition to hierarchy-aware scheduling gradual.  Scheduling
logic will be converted to use cfqg->children_weight without actually
changing the behavior.  When everything is ready,
blkcg_weight_parent() will be replaced with proper parent function.

This patch doesn't introduce any behavior chagne.

v2: s/cfqg->level_weight/cfqg->children_weight/ as per Vivek.

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

1 files changed, 76 insertions, 0 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 175218d66d67..7701c3fe49b9 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -225,6 +225,18 @@ struct cfq_group {
         u64 vdisktime;
 
         /*
+         * The number of active cfqgs and sum of their weights under this
+         * cfqg.  This covers this cfqg's leaf_weight and all children's
+         * weights, but does not cover weights of further descendants.
+         *
+         * If a cfqg is on the service tree, it's active.  An active cfqg
+         * also activates its parent and contributes to the children_weight
+         * of the parent.
+         */
+        int nr_active;
+        unsigned int children_weight;
+
+        /*
          * There are two weights - (internal) weight is the weight of this
          * cfqg against the sibling cfqgs.  leaf_weight is the wight of
          * this cfqg against the child cfqgs.  For the root cfqg, both
@@ -583,6 +595,22 @@ 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)
+{
+        struct blkcg_gq *blkg = cfqg_to_blkg(cfqg);
+        struct cfq_group *root;
+
+        while (blkg->parent)
+                blkg = blkg->parent;
+        root = blkg_to_cfqg(blkg);
+
+        return root != cfqg ? root : NULL;
+}
+
 static inline void cfqg_get(struct cfq_group *cfqg)
 {
         return blkg_get(cfqg_to_blkg(cfqg));
@@ -683,6 +711,7 @@ 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 void cfqg_get(struct cfq_group *cfqg) { }
 static inline void cfqg_put(struct cfq_group *cfqg) { }
 
@@ -1208,11 +1237,33 @@ cfq_update_group_weight(struct cfq_group *cfqg)
 static void
 cfq_group_service_tree_add(struct cfq_rb_root *st, struct cfq_group *cfqg)
 {
+        struct cfq_group *pos = cfqg;
+        bool propagate;
+
+        /* add to the service tree */
         BUG_ON(!RB_EMPTY_NODE(&cfqg->rb_node));
 
         cfq_update_group_weight(cfqg);
         __cfq_group_service_tree_add(st, cfqg);
         st->total_weight += cfqg->weight;
+
+        /*
+         * Activate @cfqg and propagate activation upwards until we meet an
+         * already activated node or reach root.
+         */
+        propagate = !pos->nr_active++;
+        pos->children_weight += pos->leaf_weight;
+
+        while (propagate) {
+                struct cfq_group *parent = cfqg_flat_parent(pos);
+
+                if (!parent)
+                        break;
+
+                propagate = !parent->nr_active++;
+                parent->children_weight += pos->weight;
+                pos = parent;
+        }
 }
 
 static void
@@ -1243,6 +1294,31 @@ cfq_group_notify_queue_add(struct cfq_data *cfqd, struct cfq_group *cfqg)
 static void
 cfq_group_service_tree_del(struct cfq_rb_root *st, struct cfq_group *cfqg)
 {
+        struct cfq_group *pos = cfqg;
+        bool propagate;
+
+        /*
+         * Undo activation from cfq_group_service_tree_add().  Deactivate
+         * @cfqg and propagate deactivation upwards.
+         */
+        propagate = !--pos->nr_active;
+        pos->children_weight -= pos->leaf_weight;
+
+        while (propagate) {
+                struct cfq_group *parent = cfqg_flat_parent(pos);
+
+                /* @pos has 0 nr_active at this point */
+                WARN_ON_ONCE(pos->children_weight);
+
+                if (!parent)
+                        break;
+
+                propagate = !--parent->nr_active;
+                parent->children_weight -= pos->weight;
+                pos = parent;
+        }
+
+        /* remove from the service tree */
         st->total_weight -= cfqg->weight;
         if (!RB_EMPTY_NODE(&cfqg->rb_node))
                 cfq_rb_erase(&cfqg->rb_node, st);