[PATCH 03/11] blk-throttle: ignore cgroup without io queued in blk_throtl_cancel_bios

[Kemeng Shi]

Ignore cgroup without io queued in blk_throtl_cancel_bios for two
reasons:
1. Save cpu cycle for trying to dispatch cgroup which is no io queued.
2. Avoid non-consistent state that cgroup is inserted to service queue
without THROTL_TG_PENDING set as tg_update_disptime will unconditional
re-insert cgroup to service queue. If we are on the default hierarchy,
IO dispatched from child in tg_dispatch_one_bio will trigger inserting
cgroup to service queue without erase first and ruin the tree.

Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxx>
---
 block/blk-throttle.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index b33bcf53b36e..acfac916ed99 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1728,7 +1728,12 @@ void blk_throtl_cancel_bios(struct gendisk *disk)
 		 * Set the flag to make sure throtl_pending_timer_fn() won't
 		 * stop until all throttled bios are dispatched.
 		 */
-		blkg_to_tg(blkg)->flags |= THROTL_TG_CANCELING;
+		tg->flags |= THROTL_TG_CANCELING;
+
+		/* No need to dispatch cgroup without io queued */
+		if (!(tg->flags & THROTL_TG_PENDING))
+			continue;
+
 		/*
 		 * Update disptime after setting the above flag to make sure
 		 * throtl_select_dispatch() won't exit without dispatching.
-- 
2.30.0