sched/numa: Convert sched_numa_balancing to a static_branch

Variable sched_numa_balancing toggles numa_balancing feature. Hence
moving from a simple read mostly variable to a more apt static_branch.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1439310261-16124-1-git-send-email-srikar@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -2114,12 +2114,16 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 #endif /* CONFIG_NUMA_BALANCING */
 }
 
+DEFINE_STATIC_KEY_FALSE(sched_numa_balancing);
+
 #ifdef CONFIG_NUMA_BALANCING
-__read_mostly bool sched_numa_balancing;
 
 void set_numabalancing_state(bool enabled)
 {
-	sched_numa_balancing = enabled;
+	if (enabled)
+		static_branch_enable(&sched_numa_balancing);
+	else
+		static_branch_disable(&sched_numa_balancing);
 }
 
 #ifdef CONFIG_PROC_SYSCTL
@@ -2128,7 +2132,7 @@ int sysctl_numa_balancing(struct ctl_table *table, int write,
 {
 	struct ctl_table t;
 	int err;
-	int state = sched_numa_balancing;
+	int state = static_branch_likely(&sched_numa_balancing);
 
 	if (write && !capable(CAP_SYS_ADMIN))
 		return -EPERM;

kernel/sched/fair.c

@@ -2069,7 +2069,7 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)
 	int local = !!(flags & TNF_FAULT_LOCAL);
 	int priv;
 
-	if (!sched_numa_balancing)
+	if (!static_branch_likely(&sched_numa_balancing))
 		return;
 
 	/* for example, ksmd faulting in a user's mm */
@@ -5562,7 +5562,7 @@ static int migrate_degrades_locality(struct task_struct *p, struct lb_env *env)
 	unsigned long src_faults, dst_faults;
 	int src_nid, dst_nid;
 
-	if (!sched_numa_balancing)
+	if (!static_branch_likely(&sched_numa_balancing))
 		return -1;
 
 	if (!p->numa_faults || !(env->sd->flags & SD_NUMA))
@@ -7874,7 +7874,7 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
 		entity_tick(cfs_rq, se, queued);
 	}
 
-	if (sched_numa_balancing)
+	if (!static_branch_unlikely(&sched_numa_balancing))
 		task_tick_numa(rq, curr);
 }
 

kernel/sched/sched.h

@@ -1003,11 +1003,7 @@ extern struct static_key sched_feat_keys[__SCHED_FEAT_NR];
 #define sched_feat(x) (sysctl_sched_features & (1UL << __SCHED_FEAT_##x))
 #endif /* SCHED_DEBUG && HAVE_JUMP_LABEL */
 
-#ifdef CONFIG_NUMA_BALANCING
-extern bool sched_numa_balancing;
-#else
-#define sched_numa_balancing (0)
-#endif /* CONFIG_NUMA_BALANCING */
+extern struct static_key_false sched_numa_balancing;
 
 static inline u64 global_rt_period(void)
 {