[RFC PATCH 1/3] sched: Make migrate_task_to() take any task

[David Vernet]

The migrate_task_to() function exposed from kernel/sched/core.c migrates
the current task, which is silently assumed to also be its first
argument, to the specified CPU. The function uses stop_one_cpu() to
migrate the task to the target CPU, which won't work if @p is not the
current task as the stop_one_cpu() callback isn't invoked on remote
CPUs.

While this operation is useful for task_numa_migrate() in fair.c, it
would be useful if __migrate_task() in core.c was given external
linkage, as it actually can be used to migrate any task to a CPU.

This patch therefore:
1. Renames the existing migrate_task_to() be called
   numa_migrate_current_task_to().
2. Renames __migrate_task() to migrate_task_to(), gives it global
   linkage, and updates all callers accordingly.

A follow-on patch will call the new migrate_task_to() from fair.c when
migrating a task in a shared wakequeue to a remote CPU.

Signed-off-by: David Vernet <void@xxxxxxxxxxxxx>
---
 kernel/sched/core.c  | 16 ++++++++--------
 kernel/sched/fair.c  |  2 +-
 kernel/sched/sched.h |  4 +++-
 3 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ac38225e6d09..d911b0631e7b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2539,8 +2539,8 @@ struct set_affinity_pending {
  * So we race with normal scheduler movements, but that's OK, as long
  * as the task is no longer on this CPU.
  */
-static struct rq *__migrate_task(struct rq *rq, struct rq_flags *rf,
-				 struct task_struct *p, int dest_cpu)
+struct rq *migrate_task_to(struct rq *rq, struct rq_flags *rf,
+			   struct task_struct *p, int dest_cpu)
 {
 	/* Affinity changed (again). */
 	if (!is_cpu_allowed(p, dest_cpu))
@@ -2573,7 +2573,7 @@ static int migration_cpu_stop(void *data)
 	local_irq_save(rf.flags);
 	/*
 	 * We need to explicitly wake pending tasks before running
-	 * __migrate_task() such that we will not miss enforcing cpus_ptr
+	 * migrate_task_to() such that we will not miss enforcing cpus_ptr
 	 * during wakeups, see set_cpus_allowed_ptr()'s TASK_WAKING test.
 	 */
 	flush_smp_call_function_queue();
@@ -2605,12 +2605,12 @@ static int migration_cpu_stop(void *data)
 		}
 
 		if (task_on_rq_queued(p))
-			rq = __migrate_task(rq, &rf, p, arg->dest_cpu);
+			rq = migrate_task_to(rq, &rf, p, arg->dest_cpu);
 		else
 			p->wake_cpu = arg->dest_cpu;
 
 		/*
-		 * XXX __migrate_task() can fail, at which point we might end
+		 * XXX migrate_task_to() can fail, at which point we might end
 		 * up running on a dodgy CPU, AFAICT this can only happen
 		 * during CPU hotplug, at which point we'll get pushed out
 		 * anyway, so it's probably not a big deal.
@@ -3259,7 +3259,7 @@ void force_compatible_cpus_allowed_ptr(struct task_struct *p)
 	alloc_cpumask_var(&new_mask, GFP_KERNEL);
 
 	/*
-	 * __migrate_task() can fail silently in the face of concurrent
+	 * migrate_task_to() can fail silently in the face of concurrent
 	 * offlining of the chosen destination CPU, so take the hotplug
 	 * lock to ensure that the migration succeeds.
 	 */
@@ -9359,7 +9359,7 @@ bool sched_smp_initialized __read_mostly;
 
 #ifdef CONFIG_NUMA_BALANCING
 /* Migrate current task p to target_cpu */
-int migrate_task_to(struct task_struct *p, int target_cpu)
+int numa_migrate_current_task_to(struct task_struct *p, int target_cpu)
 {
 	struct migration_arg arg = { p, target_cpu };
 	int curr_cpu = task_cpu(p);
@@ -9439,7 +9439,7 @@ static int __balance_push_cpu_stop(void *arg)
 
 	if (task_rq(p) == rq && task_on_rq_queued(p)) {
 		cpu = select_fallback_rq(rq->cpu, p);
-		rq = __migrate_task(rq, &rf, p, cpu);
+		rq = migrate_task_to(rq, &rf, p, cpu);
 	}
 
 	rq_unlock(rq, &rf);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6189d1a45635..292c593fc84f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2278,7 +2278,7 @@ static int task_numa_migrate(struct task_struct *p)
 
 	best_rq = cpu_rq(env.best_cpu);
 	if (env.best_task == NULL) {
-		ret = migrate_task_to(p, env.best_cpu);
+		ret = numa_migrate_current_task_to(p, env.best_cpu);
 		WRITE_ONCE(best_rq->numa_migrate_on, 0);
 		if (ret != 0)
 			trace_sched_stick_numa(p, env.src_cpu, NULL, env.best_cpu);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 556496c77dc2..5a86e9795731 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1718,7 +1718,7 @@ enum numa_faults_stats {
 	NUMA_CPUBUF
 };
 extern void sched_setnuma(struct task_struct *p, int node);
-extern int migrate_task_to(struct task_struct *p, int cpu);
+extern int numa_migrate_current_task_to(struct task_struct *p, int target_cpu);
 extern int migrate_swap(struct task_struct *p, struct task_struct *t,
 			int cpu, int scpu);
 extern void init_numa_balancing(unsigned long clone_flags, struct task_struct *p);
@@ -1731,6 +1731,8 @@ init_numa_balancing(unsigned long clone_flags, struct task_struct *p)
 
 #ifdef CONFIG_SMP
 
+extern struct rq *migrate_task_to(struct rq *rq, struct rq_flags *rf,
+				  struct task_struct *p, int dest_cpu);
 static inline void
 queue_balance_callback(struct rq *rq,
 		       struct balance_callback *head,
-- 
2.40.1