[PATCH v3 2/2] sched/uclamp: Add uclamp_is_used() check before enable it

[Xuewen Yan]

Because the static_branch_enable() would get the cpus_read_lock(),
and sometimes users may frequently set the uclamp value of tasks,
and this operation would call the static_branch_enable()
frequently, so add the uclamp_is_used() check to prevent calling
the cpus_read_lock() frequently.
And to make the code more concise, add a helper function to encapsulate
this and use it everywhere we enable sched_uclamp_used.

Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Reviewed-by: Christian Loehle <christian.loehle@xxxxxxx>
---
V3:
- update comment for helper function (Vincent)
- add Reviewed-by
---
V2:
- change some commit message;
- Add a helper function and use it everywhere when enable uclamp (Vincent)
---
---
 kernel/sched/core.c     |  6 +++---
 kernel/sched/sched.h    | 13 +++++++++++++
 kernel/sched/syscalls.c |  2 +-
 3 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 38a7192bfc19..0466a2f61b99 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1942,12 +1942,12 @@ static int sysctl_sched_uclamp_handler(const struct ctl_table *table, int write,
 	}
 
 	if (update_root_tg) {
-		static_branch_enable(&sched_uclamp_used);
+		sched_uclamp_enable();
 		uclamp_update_root_tg();
 	}
 
 	if (old_min_rt != sysctl_sched_uclamp_util_min_rt_default) {
-		static_branch_enable(&sched_uclamp_used);
+		sched_uclamp_enable();
 		uclamp_sync_util_min_rt_default();
 	}
 
@@ -9295,7 +9295,7 @@ static ssize_t cpu_uclamp_write(struct kernfs_open_file *of, char *buf,
 	if (req.ret)
 		return req.ret;
 
-	static_branch_enable(&sched_uclamp_used);
+	sched_uclamp_enable();
 
 	guard(mutex)(&uclamp_mutex);
 	guard(rcu)();
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 3624fdce5536..98fe2c8f6961 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3407,6 +3407,17 @@ static inline bool uclamp_is_used(void)
 	return static_branch_likely(&sched_uclamp_used);
 }
 
+/*
+ * Enabling static branches would get the cpus_read_lock(),
+ * check uclamp_is_used before enabling it. There is no race
+ * issue because we never disable this static key once enabled.
+ */
+static inline void sched_uclamp_enable(void)
+{
+	if (!uclamp_is_used())
+		static_branch_enable(&sched_uclamp_used);
+}
+
 static inline unsigned long uclamp_rq_get(struct rq *rq,
 					  enum uclamp_id clamp_id)
 {
@@ -3486,6 +3497,8 @@ static inline bool uclamp_is_used(void)
 	return false;
 }
 
+static inline void sched_uclamp_enable(void) {}
+
 static inline unsigned long
 uclamp_rq_get(struct rq *rq, enum uclamp_id clamp_id)
 {
diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 456d339be98f..9100a77e9d79 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -368,7 +368,7 @@ static int uclamp_validate(struct task_struct *p,
 	 * blocking operation which obviously cannot be done while holding
 	 * scheduler locks.
 	 */
-	static_branch_enable(&sched_uclamp_used);
+	sched_uclamp_enable();
 
 	return 0;
 }
-- 
2.25.1