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

Frederic Weisbecker
2024-05-26T20:57:07Z

Le Sat, May 25, 2024 at 12:06:06AM +0200, Thomas Gleixner a écrit :
> On Fri, May 24 2024 at 20:37, Oleg Nesterov wrote:
>
> > I've already had a few beers today, I know I'll regret about this
> > email tomorrow, but I can't resist ;)
>
> You won't regret it. :)
>
> > On 05/24, Frederic Weisbecker wrote:
> > But again, again. tick_sched_do_timer() says
> >
> > * If nohz_full is enabled, this should not happen because the
> > * 'tick_do_timer_cpu' CPU never relinquishes.
> >
> > so I guess it is not supposed to happen?
>
> 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. But even on S390 which has
> a truly usable and useful clocksource the tick stays periodic to begin
> with. Why?
>
> The NOHZ ready notification happens late in the boot process via:
> fs_initcall(clocksource_done_booting)
>
> So by the time that happens, the secondary CPUs are up and have taken
> over the do timer duty.
>
> [ 0.600381] smp: Bringing up secondary CPUs ...
>
> ....
>
> [ 1.917842] clocksource: Switched to clocksource kvm-clock
> [ 1.918548] clocksource_done_booting: Switched to NOHZ // debug printk
>
> This is the point where tick_nohz_activate() is called first time and
> that does:
>
> tick_sched_flag_set(ts, TS_FLAG_NOHZ);
>
> So up to this point the tick is never stopped neither on housekeeping
> nor on NOHZ FULL CPUs:
>
> tick_nohz_full_update_tick()
> if (!tick_sched_flag_test(ts, TS_FLAG_NOHZ))
> return;

Oh nice! I indeed completely skipped the clocksource part of the picture!

Thanks.