delayacct: Add static_branch in scheduler hooks

Cheaper when delayacct is disabled.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lkml.kernel.org/r/20210505111525.248028369@infradead.org

include/linux/delayacct.h

@@ -58,8 +58,10 @@ struct task_delay_info {
 
 #include <linux/sched.h>
 #include <linux/slab.h>
+#include <linux/jump_label.h>
 
 #ifdef CONFIG_TASK_DELAY_ACCT
+DECLARE_STATIC_KEY_TRUE(delayacct_key);
 extern int delayacct_on;	/* Delay accounting turned on/off */
 extern struct kmem_cache *delayacct_cache;
 extern void delayacct_init(void);
@@ -114,6 +116,9 @@ static inline void delayacct_tsk_free(struct task_struct *tsk)
 
 static inline void delayacct_blkio_start(void)
 {
+	if (!static_branch_likely(&delayacct_key))
+		return;
+
 	delayacct_set_flag(current, DELAYACCT_PF_BLKIO);
 	if (current->delays)
 		__delayacct_blkio_start();
@@ -121,6 +126,9 @@ static inline void delayacct_blkio_start(void)
 
 static inline void delayacct_blkio_end(struct task_struct *p)
 {
+	if (!static_branch_likely(&delayacct_key))
+		return;
+
 	if (p->delays)
 		__delayacct_blkio_end(p);
 	delayacct_clear_flag(p, DELAYACCT_PF_BLKIO);

kernel/delayacct.c

@@ -14,6 +14,7 @@
 #include <linux/delayacct.h>
 #include <linux/module.h>
 
+DEFINE_STATIC_KEY_TRUE(delayacct_key);
 int delayacct_on __read_mostly = 1;	/* Delay accounting turned on/off */
 struct kmem_cache *delayacct_cache;
 
@@ -28,6 +29,8 @@ void delayacct_init(void)
 {
 	delayacct_cache = KMEM_CACHE(task_delay_info, SLAB_PANIC|SLAB_ACCOUNT);
 	delayacct_tsk_init(&init_task);
+	if (!delayacct_on)
+		static_branch_disable(&delayacct_key);
 }
 
 void __delayacct_tsk_init(struct task_struct *tsk)