Re: sched/isolation: tick_take_do_timer_from_boot() calls smp_call_function_single() with irqs disabled

Oleg Nesterov
2024-05-27T15:59:14Z

On 05/27, Nicholas Piggin wrote:
>
> On Mon May 27, 2024 at 5:27 AM AEST, Oleg Nesterov wrote:
> > The more I grep the more I confused.
> >
> > On 05/25, Thomas Gleixner wrote:
> > >
> > > Right. It does not happen because the kernel starts with jiffies as
> > > clocksource except on S390. The jiffies clocksource is not qualified to
> > > switch over to NOHZ mode for obvious reasons.
> >
> > Not obvious for those who never looked at this code ;)
> >
> > OK, clocksource_jiffies doesn't have CLOCK_SOURCE_VALID_FOR_HRES,
>
> jiffies clocksource requires a timer to run it I suppose.

I meant, this is enough to ensure that clocksource_done_booting() paths should
find another cs, best != curr_clocksource, and call timekeeping_notify().

Nevermind, quite possibly I misread this code.

> > And I still don't understand why we can rely on can_stop_idle_tick() even
> > in tick_nohz_idle_stop_tick().
>
> AFAIKS timer_expires_base would be 0 unless tick_nohz_next_event()
> were called, but that is only called from places that checked
> can_stop_idle_tick() or is already a tick_nohz_full() CPU.

OK, thanks.

So, Frederic, Nicholas, any objections to the trivial change below?

We can cleanup the tick_do_timer_boot_cpu logic on top of it.

Oleg.

--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -178,26 +178,6 @@ void tick_setup_periodic(struct clock_event_device *dev, int broadcast)
}
}

-#ifdef CONFIG_NO_HZ_FULL
-static void giveup_do_timer(void *info)
-{
- int cpu = *(unsigned int *)info;
-
- WARN_ON(tick_do_timer_cpu != smp_processor_id());
-
- tick_do_timer_cpu = cpu;
-}
-
-static void tick_take_do_timer_from_boot(void)
-{
- int cpu = smp_processor_id();
- int from = tick_do_timer_boot_cpu;
-
- if (from >= 0 && from != cpu)
- smp_call_function_single(from, giveup_do_timer, &cpu, 1);
-}
-#endif
-
/*
* Setup the tick device
*/
@@ -231,9 +211,8 @@ static void tick_setup_device(struct tick_device *td,

} else if (tick_do_timer_boot_cpu != -1 &&
!tick_nohz_full_cpu(cpu)) {
- tick_take_do_timer_from_boot();
tick_do_timer_boot_cpu = -1;
- WARN_ON(READ_ONCE(tick_do_timer_cpu) != cpu);
+ WRITE_ONCE(tick_do_timer_cpu, cpu);
#endif
}