Re: [PATCH] sched: Prevent raising SCHED_SOFTIRQ when CPU is !active

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 15 Dec 2020 15:23:39 +0100 (CET)
Anna-Maria Behnsen <anna-maria@linutronix.de> wrote:

> > > +	/*
> > > +	 * Remove CPU from nohz.idle_cpus_mask to prevent participating in
> > > +	 * load balancing when not active
> > > +	 */
> > > +	nohz_balance_exit_idle(rq);
> > > +
> > >  	set_cpu_active(cpu, false);
> > >  	/*
> > >  	 * We've cleared cpu_active_mask, wait for all preempt-disabled and RCU  
> > 
> > OK, so we must clear the state before !active, because getting an
> > interrupt/softirq after would trigger the badness. And we're guaranteed
> > nothing blocks between them to re-set it.  
> 
> As far as I understood, it is not a problem whether the delete is before or
> after !active. When it is deleted after, the remote CPU will return in
> kick_ilb() because cpu is not idle, because it is running the hotplug
> thread.

I was thinking that disabling it after may also cause some badness. Even if
it does not, I think there's no harm in clearing it just before setting cpu
active to false. And I find that the safer option.

-- Steve