Re: [PATCH] init/main: Clear boot task idle flag

[Liam R. Howlett]

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> [230913 09:53]:
> On Tue, Sep 12, 2023 at 08:56:47PM -0400, Liam R. Howlett wrote:
> 
> > diff --git a/init/main.c b/init/main.c
> > index ad920fac325c..f74772acf612 100644
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -696,7 +696,7 @@ noinline void __ref __noreturn rest_init(void)
> >  	 */
> >  	rcu_read_lock();
> >  	tsk = find_task_by_pid_ns(pid, &init_pid_ns);
> > -	tsk->flags |= PF_NO_SETAFFINITY;
> > +	tsk->flags |= PF_NO_SETAFFINITY | PF_IDLE;
> >  	set_cpus_allowed_ptr(tsk, cpumask_of(smp_processor_id()));
> >  	rcu_read_unlock();
> >  
> 
> Hmm, isn't that pid-1 you're setting PF_IDLE on?

Yes, thanks.  I think that is what Geert is hitting with my patch.

debug __might_resched() in kernel/sched/core.c is failing to return in
that first (complex) if statement.  His report says pid 1 so this is
likely the issue.

> 
> The task becoming idle is 'current' at this point, see the
> cpu_startup_entry() call below.
> 
> Would not something like so be the right thing?
> 
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2299a5cfbfb9..802551e0009b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -9269,7 +9269,7 @@ void __init init_idle(struct task_struct *idle, int cpu)
>  	 * PF_KTHREAD should already be set at this point; regardless, make it
>  	 * look like a proper per-CPU kthread.
>  	 */
> -	idle->flags |= PF_IDLE | PF_KTHREAD | PF_NO_SETAFFINITY;
> +	idle->flags |= PF_KTHREAD | PF_NO_SETAFFINITY;

I am concerned this will alter more than just the current task, which
would mean more modifications later.  There is a comment about it being
called 'more than once' and 'per cpu' so I am hesitant to change the
function itself.

Although I am unsure of the call path.. fork_idle() -> init_idle() I
guess?

>  	kthread_set_per_cpu(idle, cpu);
>  
>  #ifdef CONFIG_SMP
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 342f58a329f5..5007b25c5bc6 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -373,6 +373,7 @@ EXPORT_SYMBOL_GPL(play_idle_precise);
>  
>  void cpu_startup_entry(enum cpuhp_state state)
>  {
> +	current->flags |= PF_IDLE;
>  	arch_cpu_idle_prepare();
>  	cpuhp_online_idle(state);
>  	while (1)