Re: [PATCH] tick/nohz_full: don't abuse smp_call_function_single() in tick_setup_device()

Frederic Weisbecker
2024-06-02T21:29:16Z

Le Sat, Jun 01, 2024 at 04:03:22PM +0200, Oleg Nesterov a écrit :
> Hi Frederic,
>
> First of all, can we please make the additional changes you suggest on top of
> this patch? I'd prefer to keep it as simple as possible, I will need to backport
> it and I'd like to simplify the internal review.

Sure!

>
> On 05/30, Frederic Weisbecker wrote:
> >
> > And after all, pushing a bit further your subsequent patch, can we get rid of
> > tick_do_timer_boot_cpu and ifdefery altogether? Such as:
>
> Sure, I thought about this from the very beginning, see
> https://lore.kernel.org/all/20240525135120.GA24152@redhat.com/
> and the changelog in
> [PATCH] tick/nohz_full: turn tick_do_timer_boot_cpu into boot_cpu_is_nohz_full
> https://lore.kernel.org/all/20240530124032.GA26833@redhat.com/
> on top of this patch.
>
> And yes, in this case it is better to check that tick_do_timer_cpu != _NONE to
> ensure that tick_nohz_full_cpu(tick_cpu) can't crash.
>
> So I considered the change which is very close to yours, except
>
> > + } else if (timekeeper == TICK_DO_TIMER_NONE) {
> > + if (WARN_ON_ONCE(tick_nohz_full_enabled()))
> > + WRITE_ONCE(tick_do_timer_cpu, cpu);
>
> I don't think we need to change tick_do_timer_cpu in this case.
> And I am not sure we need to check tick_nohz_full_enabled() here.
> IOW, I was thinking about

Hmm, in case of cpu-hotplug operations (that is after boot), we may be
past nohz enablement and therefore it might be TICK_DO_TIMER_NONE.

>
> if (!td->evtdev) {
> int tick_cpu = READ_ONCE(tick_do_timer_cpu);
> /*
> * If no cpu took the do_timer update, assign it to
> * this cpu:
> */
> if (tick_cpu == TICK_DO_TIMER_BOOT) {
> WRITE_ONCE(tick_do_timer_cpu, cpu);
> tick_next_period = ktime_get();
> /*
> * The boot CPU may be nohz_full, in which case the
> * first housekeeping secondary will take do_timer()
> * from us.
> */
> } else if (!WARN_ON_ONCE(tick_cpu == TICK_DO_TIMER_NONE)) &&
> tick_nohz_full_cpu(tick_cpu) &&
> !tick_nohz_full_cpu(cpu)) {
> /*
> * The boot CPU will stay in periodic (NOHZ disabled)
> * mode until clocksource_done_booting() called after
> * smp_init() selects a high resolution clocksource and
> * timekeeping_notify() kicks the NOHZ stuff alive.
> *
> * So this WRITE_ONCE can only race with the READ_ONCE
> * check in tick_periodic() but this race is harmless.
> */
> WRITE_ONCE(tick_do_timer_cpu, cpu);
> }
>
> But you know, somehow I like
> [PATCH] tick/nohz_full: turn tick_do_timer_boot_cpu into boot_cpu_is_nohz_full
> https://lore.kernel.org/all/20240530124032.GA26833@redhat.com/
> a bit more, to me the code looks more understandable this way.
>
> Note that this patch doesn't really need to keep #ifdef CONFIG_NO_HZ_FULL,
>
> if (!td->evtdev) {
> static bool boot_cpu_is_nohz_full;
> /*
> * If no cpu took the do_timer update, assign it to
> * this cpu:
> */
> if (READ_ONCE(tick_do_timer_cpu) == TICK_DO_TIMER_BOOT) {
> WRITE_ONCE(tick_do_timer_cpu, cpu);
> tick_next_period = ktime_get();
> /*
> * The boot CPU may be nohz_full, in which case the
> * first housekeeping secondary will take do_timer()
> * from us.
> */
> boot_cpu_is_nohz_full = tick_nohz_full_cpu(cpu);
> } else if (boot_cpu_is_nohz_full && !tick_nohz_full_cpu(cpu)) {
> boot_cpu_is_nohz_full = false;
> /*
> * The boot CPU will stay in periodic (NOHZ disabled)
> * mode until clocksource_done_booting() called after
> * smp_init() selects a high resolution clocksource and
> * timekeeping_notify() kicks the NOHZ stuff alive.
> *
> * So this WRITE_ONCE can only race with the READ_ONCE
> * check in tick_periodic() but this race is harmless.
> */
> WRITE_ONCE(tick_do_timer_cpu, cpu);
> }
>
> should work without #ifdef.
>
> In this case I don't think we need the _NONE check, tick_sched_do_timer() will
> complain.

Right...

>
> But I won't argue. I will be happy to make V2 which follows your recommendations
> but again, can I do this on top of this patch?

I guess the static version above should work to remove the ifdef. And yes on top is fine.

Thanks!