linux-kernel - Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance

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Date:   Thu, 11 Mar 2021 17:42:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     tglx@...utronix.de, mingo@...nel.org, bigeasy@...utronix.de,
        swood@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vincent.donnefort@....com, qais.yousef@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs
 hotplug-rollback

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.