Re: [PATCH 4/4] Revert "kernel/sched: Modify initial boot task idle setup"

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Oct 20, 2023 at 02:31:13PM +0200, Frederic Weisbecker wrote:

> Yeah I can't say I really like the old coverage of PF_IDLE either. The new one
> (after Liam's patch) is only halfway better defined though: it makes the boot
> CPU's idle behave quite well: PF_IDLE is set on idle entry. And secondary
> CPU's idle behave quite well also except when they go offline and then online
> again. And then the secondary boot code becomes PF_IDLE.

Bah offline, yeah, we should just not do that :-)

> We probably need something like this:
> 
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 3b9d5c7eb4a2..b24d7937b989 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1394,7 +1394,9 @@ void cpuhp_report_idle_dead(void)
>  {
>  	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
>  
> +	current->flags &= ~PF_IDLE;
>  	BUG_ON(st->state != CPUHP_AP_OFFLINE);
> +
>  	rcutree_report_cpu_dead();
>  	st->state = CPUHP_AP_IDLE_DEAD;
>  	/*
> @@ -1642,6 +1644,8 @@ void cpuhp_online_idle(enum cpuhp_state state)
>  {
>  	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
>  
> +	current->flags |= PF_IDLE;
> +
>  	/* Happens for the boot cpu */
>  	if (state != CPUHP_AP_ONLINE_IDLE)
>  		return;

Yeah that works I suppose.