sched: Make the idle task quack like a per-CPU kthread

For all intents and purposes, the idle task is a per-CPU kthread. It isn't
created via the same route as other pcpu kthreads however, and as a result
it is missing a few bells and whistles: it fails kthread_is_per_cpu() and
it doesn't have PF_NO_SETAFFINITY set.

Fix the former by giving the idle task a kthread struct along with the
KTHREAD_IS_PER_CPU flag. This requires some extra iffery as init_idle()
call be called more than once on the same idle task.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210510151024.2448573-2-valentin.schneider@arm.com

1 files changed, 18 insertions, 12 deletions

diff --git a/kernel/kthread.c b/kernel/kthread.c
index fe3f2a40d61e..3d326833092b 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -68,16 +68,6 @@ enum KTHREAD_BITS {
 	KTHREAD_SHOULD_PARK,
 };
 
-static inline void set_kthread_struct(void *kthread)
-{
-	/*
-	 * We abuse ->set_child_tid to avoid the new member and because it
-	 * can't be wrongly copied by copy_process(). We also rely on fact
-	 * that the caller can't exec, so PF_KTHREAD can't be cleared.
-	 */
-	current->set_child_tid = (__force void __user *)kthread;
-}
-
 static inline struct kthread *to_kthread(struct task_struct *k)
 {
 	WARN_ON(!(k->flags & PF_KTHREAD));
@@ -103,6 +93,22 @@ static inline struct kthread *__to_kthread(struct task_struct *p)
 	return kthread;
 }
 
+void set_kthread_struct(struct task_struct *p)
+{
+	struct kthread *kthread;
+
+	if (__to_kthread(p))
+		return;
+
+	kthread = kzalloc(sizeof(*kthread), GFP_KERNEL);
+	/*
+	 * We abuse ->set_child_tid to avoid the new member and because it
+	 * can't be wrongly copied by copy_process(). We also rely on fact
+	 * that the caller can't exec, so PF_KTHREAD can't be cleared.
+	 */
+	p->set_child_tid = (__force void __user *)kthread;
+}
+
 void free_kthread_struct(struct task_struct *k)
 {
 	struct kthread *kthread;
@@ -272,8 +278,8 @@ static int kthread(void *_create)
 	struct kthread *self;
 	int ret;
 
-	self = kzalloc(sizeof(*self), GFP_KERNEL);
-	set_kthread_struct(self);
+	set_kthread_struct(current);
+	self = to_kthread(current);
 
 	/* If user was SIGKILLed, I release the structure. */
 	done = xchg(&create->done, NULL);