blk-throttle: Move service tree validation out of the throtl_rb_first()

The throtl_schedule_next_dispatch() will validate if the service queue
is empty before calling update_min_dispatch_time(), and the
update_min_dispatch_time() will call throtl_rb_first(), which will
validate service queue again.

Thus we can move the service queue validation out of the
throtl_rb_first() to remove the redundant validation in the fast path.

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index f1bcb5c..38aed8b 100644 (file)
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -638,9 +638,6 @@ static struct throtl_grp *
 throtl_rb_first(struct throtl_service_queue *parent_sq)
 {
        struct rb_node *n;
-       /* Service tree is empty */
-       if (!parent_sq->nr_pending)
-               return NULL;
 
        n = rb_first_cached(&parent_sq->pending_tree);
        WARN_ON_ONCE(!n);
@@ -1224,9 +1221,13 @@ static int throtl_select_dispatch(struct throtl_service_queue *parent_sq)
        unsigned int nr_disp = 0;
 
        while (1) {
-               struct throtl_grp *tg = throtl_rb_first(parent_sq);
+               struct throtl_grp *tg;
                struct throtl_service_queue *sq;
 
+               if (!parent_sq->nr_pending)
+                       break;
+
+               tg = throtl_rb_first(parent_sq);
                if (!tg)
                        break;