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

Steven Rostedt
2020-12-15T14:35:54Z

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