blkcg: add request_queue->root_blkg

With per-queue policy activation, root blkg creation will be moved to
blkcg core.  Add q->root_blkg in preparation.  For blk-throtl, this
replaces throtl_data->root_tg; however, cfq needs to keep
cfqd->root_group for !CONFIG_CFQ_GROUP_IOSCHED.

This is to prepare for per-queue policy activation and doesn't cause
any functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 3 insertions, 1 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index de95f9a2acf8..86440e04f3ee 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3964,8 +3964,10 @@ static int cfq_init_queue(struct request_queue *q)
         spin_lock_irq(q->queue_lock);
 
         blkg = blkg_lookup_create(&blkio_root_cgroup, q, true);
-        if (!IS_ERR(blkg))
+        if (!IS_ERR(blkg)) {
+                q->root_blkg = blkg;
                 cfqd->root_group = blkg_to_cfqg(blkg);
+        }
 
         spin_unlock_irq(q->queue_lock);
         rcu_read_unlock();