Messages in this thread | | | Date | Tue, 15 Dec 2020 16:05:29 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched: Prevent raising SCHED_SOFTIRQ when CPU is !active |
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On Tue, Dec 15, 2020 at 09:34:15AM -0500, Steven Rostedt wrote: > 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.
The paranoid in me wanted to write it like:
preempt_disable(); nohz_balance_exit_idle(rq); set_cpu_active(cpu, false); preempt_enable();
(or possibly even local_irq_disable), to guarantee we don't hit idle between them (which could re-set the nohz idle state we just cleared).
But then I gave up :-)
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