Re: kvm+nouveau induced lockdep gripe

[Mike Galbraith]

On Tue, 2020-10-27 at 10:00 +0100, Sebastian Andrzej Siewior wrote:
> On 2020-10-27 07:03:38 [+0100], Mike Galbraith wrote:
> > On Mon, 2020-10-26 at 20:53 +0100, Sebastian Andrzej Siewior wrote:
> > >
> > > Could you try this, please?
> >
> > Nogo, first call of sched_setscheduler() is via kthread_create().  I
> > confirmed that nuking that (gratuitous user foot saving override) call
> > on top of moving sched_set_fifo() does shut it up, but that won't fly.
>
> mkay. but this then, too.

Yup, might even fly.

>  Let me try if I can figure out when this
> broke.
>
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 3edaa380dc7b4..64d6afb127239 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -244,6 +244,7 @@ EXPORT_SYMBOL_GPL(kthread_parkme);
>  static int kthread(void *_create)
>  {
>  	/* Copy data: it's on kthread's stack */
> +	static const struct sched_param param = { .sched_priority = 0 };
>  	struct kthread_create_info *create = _create;
>  	int (*threadfn)(void *data) = create->threadfn;
>  	void *data = create->data;
> @@ -273,6 +274,13 @@ static int kthread(void *_create)
>  	init_completion(&self->parked);
>  	current->vfork_done = &self->exited;
>
> +	/*
> +	 * root may have changed our (kthreadd's) priority or CPU mask.
> +	 * The kernel thread should not inherit these properties.
> +	 */
> +	sched_setscheduler_nocheck(current, SCHED_NORMAL, &param);
> +	set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_KTHREAD));
> +
>  	/* OK, tell user we're spawned, wait for stop or wakeup */
>  	__set_current_state(TASK_UNINTERRUPTIBLE);
>  	create->result = current;
> @@ -370,7 +378,6 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
>  	}
>  	task = create->result;
>  	if (!IS_ERR(task)) {
> -		static const struct sched_param param = { .sched_priority = 0 };
>  		char name[TASK_COMM_LEN];
>
>  		/*
> @@ -379,13 +386,6 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
>  		 */
>  		vsnprintf(name, sizeof(name), namefmt, args);
>  		set_task_comm(task, name);
> -		/*
> -		 * root may have changed our (kthreadd's) priority or CPU mask.
> -		 * The kernel thread should not inherit these properties.
> -		 */
> -		sched_setscheduler_nocheck(task, SCHED_NORMAL, &param);
> -		set_cpus_allowed_ptr(task,
> -				     housekeeping_cpumask(HK_FLAG_KTHREAD));
>  	}
>  	kfree(create);
>  	return task;
>
> Sebastian