blkcg: make blkcg_gq's hierarchical

Currently a child blkg (blkcg_gq) can be created even if its parent
doesn't exist.  ie. Given a blkg, it's not guaranteed that its
ancestors will exist.  This makes it difficult to implement proper
hierarchy support for blkcg policies.

Always create blkgs recursively and make a child blkg hold a reference
to its parent.  blkg->parent is added so that finding the parent is
easy.  blkcg_parent() is also added in the process.

This change can be visible to userland.  e.g. while issuing IO in a
nested cgroup didn't affect the ancestors at all, now it will
initialize all ancestor blkgs and zero stats for the request_queue
will always appear on them.  While this is userland visible, this
shouldn't cause any functional difference.

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

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 18ae480..942f344 100644 (file)
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -201,7 +201,16 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
        }
        blkg = new_blkg;
 
-       /* insert */
+       /* link parent and insert */
+       if (blkcg_parent(blkcg)) {
+               blkg->parent = __blkg_lookup(blkcg_parent(blkcg), q, false);
+               if (WARN_ON_ONCE(!blkg->parent)) {
+                       blkg = ERR_PTR(-EINVAL);
+                       goto err_put_css;
+               }
+               blkg_get(blkg->parent);
+       }
+
        spin_lock(&blkcg->lock);
        ret = radix_tree_insert(&blkcg->blkg_tree, q->id, blkg);
        if (likely(!ret)) {
@@ -213,6 +222,10 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
        if (!ret)
                return blkg;
 
+       /* @blkg failed fully initialized, use the usual release path */
+       blkg_put(blkg);
+       return ERR_PTR(ret);
+
 err_put_css:
        css_put(&blkcg->css);
 err_free_blkg:
@@ -226,8 +239,9 @@ err_free_blkg:
  * @q: request_queue of interest
  *
  * Lookup blkg for the @blkcg - @q pair.  If it doesn't exist, try to
- * create one.  This function should be called under RCU read lock and
- * @q->queue_lock.
+ * create one.  blkg creation is performed recursively from blkcg_root such
+ * that all non-root blkg's have access to the parent blkg.  This function
+ * should be called under RCU read lock and @q->queue_lock.
  *
  * Returns pointer to the looked up or created blkg on success, ERR_PTR()
  * value on error.  If @q is dead, returns ERR_PTR(-EINVAL).  If @q is not
@@ -252,7 +266,23 @@ struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
        if (blkg)
                return blkg;
 
-       return blkg_create(blkcg, q, NULL);
+       /*
+        * Create blkgs walking down from blkcg_root to @blkcg, so that all
+        * non-root blkgs have access to their parents.
+        */
+       while (true) {
+               struct blkcg *pos = blkcg;
+               struct blkcg *parent = blkcg_parent(blkcg);
+
+               while (parent && !__blkg_lookup(parent, q, false)) {
+                       pos = parent;
+                       parent = blkcg_parent(parent);
+               }
+
+               blkg = blkg_create(pos, q, NULL);
+               if (pos == blkcg || IS_ERR(blkg))
+                       return blkg;
+       }
 }
 EXPORT_SYMBOL_GPL(blkg_lookup_create);
 
@@ -321,8 +351,10 @@ static void blkg_rcu_free(struct rcu_head *rcu_head)
 
 void __blkg_release(struct blkcg_gq *blkg)
 {
-       /* release the extra blkcg reference this blkg has been holding */
+       /* release the blkcg and parent blkg refs this blkg has been holding */
        css_put(&blkg->blkcg->css);
+       if (blkg->parent)
+               blkg_put(blkg->parent);
 
        /*
         * A group is freed in rcu manner. But having an rcu lock does not

diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index 2459730..b26ed58 100644 (file)
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -94,8 +94,13 @@ struct blkcg_gq {
        struct list_head                q_node;
        struct hlist_node               blkcg_node;
        struct blkcg                    *blkcg;
+
+       /* all non-root blkcg_gq's are guaranteed to have access to parent */
+       struct blkcg_gq                 *parent;
+
        /* request allocation list for this blkcg-q pair */
        struct request_list             rl;
+
        /* reference count */
        int                             refcnt;
 
@@ -181,6 +186,19 @@ static inline struct blkcg *bio_blkcg(struct bio *bio)
 }
 
 /**
+ * blkcg_parent - get the parent of a blkcg
+ * @blkcg: blkcg of interest
+ *
+ * Return the parent blkcg of @blkcg.  Can be called anytime.
+ */
+static inline struct blkcg *blkcg_parent(struct blkcg *blkcg)
+{
+       struct cgroup *pcg = blkcg->css.cgroup->parent;
+
+       return pcg ? cgroup_to_blkcg(pcg) : NULL;
+}
+
+/**
  * blkg_to_pdata - get policy private data
  * @blkg: blkg of interest
  * @pol: policy of interest