[PATCH] blk-cgroup: don't update the blkg lookup hint in blkg_conf_prep

Christoph Hellwig
2022-09-27T06:54:33Z

blkg_conf_prep just creates a new blkg structure, there is no real
need to update the lookup hint which should only be done on a
successful lookup in the I/O path.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
block/blk-cgroup.c | 17 ++++-------------
1 file changed, 4 insertions(+), 13 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 94af5f3f3620b..6a5c849ee061b 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -263,14 +263,6 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct gendisk *disk,
return NULL;
}

-static void blkg_update_hint(struct blkcg *blkcg, struct blkcg_gq *blkg)
-{
- lockdep_assert_held(&blkg->q->queue_lock);
-
- if (blkcg != &blkcg_root && blkg != rcu_dereference(blkcg->blkg_hint))
- rcu_assign_pointer(blkcg->blkg_hint, blkg);
-}
-
/*
* If @new_blkg is %NULL, this function tries to allocate a new one as
* necessary using %GFP_NOWAIT. @new_blkg is always consumed on return.
@@ -383,7 +375,9 @@ static struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
spin_lock_irqsave(&q->queue_lock, flags);
blkg = blkg_lookup(blkcg, q);
if (blkg) {
- blkg_update_hint(blkcg, blkg);
+ if (blkcg != &blkcg_root &&
+ blkg != rcu_dereference(blkcg->blkg_hint))
+ rcu_assign_pointer(blkcg->blkg_hint, blkg);
goto found;
}

@@ -680,10 +674,8 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
}

blkg = blkg_lookup(blkcg, q);
- if (blkg) {
- blkg_update_hint(blkcg, blkg);
+ if (blkg)
goto success;
- }

/*
* Create blkgs walking down from blkcg_root to @blkcg, so that all
@@ -727,7 +719,6 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,

blkg = blkg_lookup(pos, q);
if (blkg) {
- blkg_update_hint(pos, blkg);
blkg_free(new_blkg);
} else {
blkg = blkg_create(pos, disk, new_blkg);
--
2.30.2