sched/fair: Move task_numa_work() init to init_numa_balancing()

We only need to set the callback_head worker function once, do it
during sched_fork().

While at it, move the comment regarding double task_work addition to
init_numa_balancing(), since the double add sentinel is first set there.
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: mgorman@suse.de
Cc: riel@surriel.com
Link: https://lkml.kernel.org/r/20190715102508.32434-3-valentin.schneider@arm.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -2495,7 +2495,7 @@ void task_numa_work(struct callback_head *work)
 
 	SCHED_WARN_ON(p != container_of(work, struct task_struct, numa_work));
 
-	work->next = work; /* protect against double add */
+	work->next = work;
 	/*
 	 * Who cares about NUMA placement when they're dying.
 	 *
@@ -2639,12 +2639,15 @@ void init_numa_balancing(unsigned long clone_flags, struct task_struct *p)
 	p->node_stamp			= 0;
 	p->numa_scan_seq		= mm ? mm->numa_scan_seq : 0;
 	p->numa_scan_period		= sysctl_numa_balancing_scan_delay;
+	/* Protect against double add, see task_tick_numa and task_numa_work */
 	p->numa_work.next		= &p->numa_work;
 	p->numa_faults			= NULL;
 	RCU_INIT_POINTER(p->numa_group, NULL);
 	p->last_task_numa_placement	= 0;
 	p->last_sum_exec_runtime	= 0;
 
+	init_task_work(&p->numa_work, task_numa_work);
+
 	/* New address space, reset the preferred nid */
 	if (!(clone_flags & CLONE_VM)) {
 		p->numa_preferred_nid = NUMA_NO_NODE;
@@ -2693,11 +2696,9 @@ static void task_tick_numa(struct rq *rq, struct task_struct *curr)
 			curr->numa_scan_period = task_scan_start(curr);
 		curr->node_stamp += period;
 
-		if (!time_before(jiffies, curr->mm->numa_next_scan)) {
-			init_task_work(work, task_numa_work); /* TODO: move this into sched_fork() */
+		if (!time_before(jiffies, curr->mm->numa_next_scan))
 			task_work_add(curr, work, true);
 	}
-	}
 }
 
 static void update_scan_period(struct task_struct *p, int new_cpu)