Re: [PATCH 4/4] sched: Exclude CPU boot code from PF_IDLE area

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 24, 2023 at 11:46:25PM +0200, Frederic Weisbecker wrote:

> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 8885be2c143e..ad18962b921d 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1945,7 +1945,7 @@ extern struct task_struct *idle_task(int cpu);
>   */
>  static __always_inline bool is_idle_task(const struct task_struct *p)
>  {
> -	return !!(p->flags & PF_IDLE);
> +	return !!(READ_ONCE(p->flags) & PF_IDLE);
>  }
>  
>  extern struct task_struct *curr_task(int cpu);
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 3b9d5c7eb4a2..3a1991010f4e 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);
>  
> +	WRITE_ONCE(current->flags, 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);
>  
> +	WRITE_ONCE(current->flags, current->flags | PF_IDLE);
> +
>  	/* Happens for the boot cpu */
>  	if (state != CPUHP_AP_ONLINE_IDLE)
>  		return;

Without changing *ALL* ->flags stores to WRITE_ONCE() I don't see the
point of this. Also, since we only care about a single bit, how does
store tearing affect things?

Not to mention if we're really paranoid, what are the SMP ordering
considerations :-)

[ also, PF_ is used for Protocol Family, Page Flag and Process Flag,
  grepping is a pain in the arse :-( ]