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    Subject[PATCH 12/24] cfq-iosched: implement cfq_group->nr_active and ->children_weight
    Date
    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>
    Cc: Vivek Goyal <vgoyal@redhat.com>
    ---
    block/cfq-iosched.c | 76 +++++++++++++++++++++++++++++++++++++++++++++++++++++
    1 file changed, 76 insertions(+)

    diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
    index 175218d6..7701c3f 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);
    --
    1.8.0.2


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