blkcg: move root blkg lookup optimization from throtl_lookup_tg() to __blkg_lookup()

Currently, both throttle and cfq policies implement their own root
blkg (blkcg_gq) lookup fast path.  This patch moves root blkg
optimization from throtl_lookup_tg() to __blkg_lookup().  cfq-iosched
currently doesn't use blkg_lookup() but will be converted and drop the
optimization too.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Arianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

2 files changed, 3 insertions, 7 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index c2c75477a6b2..1f63fc834dc3 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -452,13 +452,6 @@ static void throtl_pd_reset_stats(struct blkg_policy_data *pd)
 static struct throtl_grp *throtl_lookup_tg(struct throtl_data *td,
                                            struct blkcg *blkcg)
 {
-        /*
-         * This is the common case when there are no blkcgs.  Avoid lookup
-         * in this case
-         */
-        if (blkcg == &blkcg_root)
-                return td_root_tg(td);
-
         return blkg_to_tg(blkg_lookup(blkcg, td->queue));
 }
 
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index d5b54aa50582..0609bce69f68 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -249,6 +249,9 @@ static inline struct blkcg_gq *__blkg_lookup(struct blkcg *blkcg,
 {
         struct blkcg_gq *blkg;
 
+        if (blkcg == &blkcg_root)
+                return q->root_blkg;
+
         blkg = rcu_dereference(blkcg->blkg_hint);
         if (blkg && blkg->q == q)
                 return blkg;