blkcg: change blkg reference counting to use percpu_ref

Every bio is now associated with a blkg putting blkg_get, blkg_try_get,
and blkg_put on the hot path. Switch over the refcnt in blkg to use
percpu_ref.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 39 insertions, 2 deletions

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 120f2e2835fb..2ca7611fe274 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -81,6 +81,37 @@ static void blkg_free(struct blkcg_gq *blkg)
         kfree(blkg);
 }
 
+static void __blkg_release(struct rcu_head *rcu)
+{
+        struct blkcg_gq *blkg = container_of(rcu, struct blkcg_gq, rcu_head);
+
+        percpu_ref_exit(&blkg->refcnt);
+
+        /* release the blkcg and parent blkg refs this blkg has been holding */
+        css_put(&blkg->blkcg->css);
+        if (blkg->parent)
+                blkg_put(blkg->parent);
+
+        wb_congested_put(blkg->wb_congested);
+
+        blkg_free(blkg);
+}
+
+/*
+ * A group is RCU protected, but having an rcu lock does not mean that one
+ * can access all the fields of blkg and assume these are valid.  For
+ * example, don't try to follow throtl_data and request queue links.
+ *
+ * Having a reference to blkg under an rcu allows accesses to only values
+ * local to groups like group stats and group rate limits.
+ */
+static void blkg_release(struct percpu_ref *ref)
+{
+        struct blkcg_gq *blkg = container_of(ref, struct blkcg_gq, refcnt);
+
+        call_rcu(&blkg->rcu_head, __blkg_release);
+}
+
 /**
  * blkg_alloc - allocate a blkg
  * @blkcg: block cgroup the new blkg is associated with
@@ -107,7 +138,6 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
         blkg->q = q;
         INIT_LIST_HEAD(&blkg->q_node);
         blkg->blkcg = blkcg;
-        atomic_set(&blkg->refcnt, 1);
 
         for (i = 0; i < BLKCG_MAX_POLS; i++) {
                 struct blkcg_policy *pol = blkcg_policy[i];
@@ -207,6 +237,11 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
                 blkg_get(blkg->parent);
         }
 
+        ret = percpu_ref_init(&blkg->refcnt, blkg_release, 0,
+                              GFP_NOWAIT | __GFP_NOWARN);
+        if (ret)
+                goto err_cancel_ref;
+
         /* invoke per-policy init */
         for (i = 0; i < BLKCG_MAX_POLS; i++) {
                 struct blkcg_policy *pol = blkcg_policy[i];
@@ -239,6 +274,8 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
         blkg_put(blkg);
         return ERR_PTR(ret);
 
+err_cancel_ref:
+        percpu_ref_exit(&blkg->refcnt);
 err_put_congested:
         wb_congested_put(wb_congested);
 err_put_css:
@@ -367,7 +404,7 @@ static void blkg_destroy(struct blkcg_gq *blkg)
          * Put the reference taken at the time of creation so that when all
          * queues are gone, group can be destroyed.
          */
-        blkg_put(blkg);
+        percpu_ref_kill(&blkg->refcnt);
 }
 
 /**