linux-kernel - [PATCH 03/10] blkcg: move root blkg lookup optimization from throtl_lookup_tg() to __blkg

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Date:	Sun, 12 Jul 2015 14:00:36 -0400
From:	Tejun Heo <tj@...nel.org>
To:	axboe@...nel.dk
Cc:	linux-kernel@...r.kernel.org, kernel-team@...com,
	vgoyal@...hat.com, avanzini.arianna@...il.com,
	Tejun Heo <tj@...nel.org>
Subject: [PATCH 03/10] 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@...nel.org>
Cc: Vivek Goyal <vgoyal@...hat.com>
Cc: Arianna Avanzini <avanzini.arianna@...il.com>
---
 block/blk-throttle.c       | 7 -------
 include/linux/blk-cgroup.h | 3 +++
 2 files changed, 3 insertions(+), 7 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index c2c7547..1f63fc8 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 d5b54aa..0609bce 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;
-- 
2.4.3

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