Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs hotplug-rollback

[Peter Zijlstra]

On Thu, Mar 11, 2021 at 03:13:04PM +0000, Valentin Schneider wrote:

> >  #ifdef CONFIG_SCHED_SMT
> >  	/*
> >  	 * When going down, decrement the number of cores with SMT present.
> 
> > @@ -8206,7 +8212,7 @@ void __init sched_init(void)
> >  		rq->sd = NULL;
> >  		rq->rd = NULL;
> >  		rq->cpu_capacity = rq->cpu_capacity_orig = SCHED_CAPACITY_SCALE;
> > -		rq->balance_callback = NULL;
> > +		rq->balance_callback = &balance_push_callback;
> >  		rq->active_balance = 0;
> >  		rq->next_balance = jiffies;
> >  		rq->push_cpu = 0;
> > @@ -8253,6 +8259,7 @@ void __init sched_init(void)
> >  
> >  #ifdef CONFIG_SMP
> >  	idle_thread_set_boot_cpu();
> > +	balance_push_set(smp_processor_id(), false);
> >  #endif
> >  	init_sched_fair_class();
> >
> 
> I don't get what these two changes do - the end result is the same as
> before, no?

IIRC the idea was to initialize the offline CPUs to the same state as if
they'd been offlined.