[PATCH v2 5/5] kthread: don't abuse kthread_create_on_cpu() in __kthread_create_worker()

[Oleg Nesterov]

kthread_create_on_cpu() sets KTHREAD_IS_PER_CPU and kthread->cpu, this
only makes sense if this kthread can be parked/unparked by cpuhp code.
kthread workers never call kthread_parkme() so this has no effect.

Change __kthread_create_worker() to simply call kthread_bind(task, cpu).
The very fact that kthread_create_on_cpu() doesn't accept a generic fmt
shows that it should not be used outside of smpboot.c.

Now, the only reason we can not unexport this helper and move it into
smpboot.c is that it sets kthread->cpu and struct kthread is not exported.
And the only reason we can not kill kthread->cpu is that kthread_unpark()
is used by drivers/gpu/drm/amd/scheduler/gpu_scheduler.c and thus we can
not turn _unpark into kthread_unpark(struct smp_hotplug_thread *, cpu).

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
Tested-by: Petr Mladek <pmladek@xxxxxxxx>
---
 kernel/kthread.c | 23 ++++++++---------------
 1 file changed, 8 insertions(+), 15 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 01d2716..956495f 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -641,6 +641,7 @@ __kthread_create_worker(int cpu, unsigned int flags,
 {
 	struct kthread_worker *worker;
 	struct task_struct *task;
+	int node = -1;
 
 	worker = kzalloc(sizeof(*worker), GFP_KERNEL);
 	if (!worker)
@@ -648,25 +649,17 @@ __kthread_create_worker(int cpu, unsigned int flags,
 
 	kthread_init_worker(worker);
 
-	if (cpu >= 0) {
-		char name[TASK_COMM_LEN];
-
-		/*
-		 * kthread_create_worker_on_cpu() allows to pass a generic
-		 * namefmt in compare with kthread_create_on_cpu. We need
-		 * to format it here.
-		 */
-		vsnprintf(name, sizeof(name), namefmt, args);
-		task = kthread_create_on_cpu(kthread_worker_fn, worker,
-					     cpu, name);
-	} else {
-		task = __kthread_create_on_node(kthread_worker_fn, worker,
-						-1, namefmt, args);
-	}
+	if (cpu >= 0)
+		node = cpu_to_node(cpu);
 
+	task = __kthread_create_on_node(kthread_worker_fn, worker,
+						node, namefmt, args);
 	if (IS_ERR(task))
 		goto fail_task;
 
+	if (cpu >= 0)
+		kthread_bind(task, cpu);
+
 	worker->flags = flags;
 	worker->task = task;
 	wake_up_process(task);
-- 
2.5.0