Nohz_full on boot CPU is broken (was: Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on queue_delayed_work)

[Frederic Weisbecker]

+Cc Nick

Le Fri, Apr 05, 2024 at 04:04:49PM +0200, Oleg Nesterov a écrit :
> On 04/03, Oleg Nesterov wrote:
> >
> > > > OTOH, Documentation/timers/no_hz.rst says
> > > >
> > > > 	Therefore, the
> > > > 	boot CPU is prohibited from entering adaptive-ticks mode.  Specifying a
> > > > 	"nohz_full=" mask that includes the boot CPU will result in a boot-time
> > > > 	error message, and the boot CPU will be removed from the mask.
> > > >
> > > > and this doesn't match the reality.
> > >
> > > Don't some archs allow the boot CPU to go down too tho? If so, this doesn't
> > > really solve the problem, right?
> >
> > I do not know. But I thought about this too.
> >
> > In the context of this discussion we do not care if the boot CPU goes down.
> > But we need at least one housekeeping CPU after cpu_down(). The comment in
> > cpu_down_maps_locked() says
> >
> > 	Also keep at least one housekeeping cpu onlined
> >
> > but it checks HK_TYPE_DOMAIN, and I do not know (and it is too late for me
> > to try to read the code ;) if housekeeping.cpumasks[HK_TYPE_TIMER] can get
> > empty or not.
> 
> This nearly killed me, but I managed to convince myself we shouldn't worry
> about cpu_down().
> 
> HK_FLAG_TIMER implies HK_FLAG_TICK.
> 
> HK_FLAG_TICK implies tick_nohz_full_setup() which sets
> tick_nohz_full_mask = non_housekeeping_mask.
> 
> When tick_setup_device() is called on a housekeeping CPU it does
> 		
> 	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;
> 
> 
> 	and this sets tick_do_timer_cpu = first-housekeeping-cpu.
> 
> cpu_down(tick_do_timer_cpu) will fail, tick_nohz_cpu_down() will nack it.
> 
> So cpu_down() can't make housekeeping.cpumasks[HK_FLAG_TIMER] empty and I
> still think that the change below is the right approach.
> 
> But probably WARN_ON() in housekeeping_any_cpu() makes sense anyway.
> 
> What do you think?

Good analysis on this nasty housekeeping VS tick code. I promised so many
times to cleanup this mess but things keep piling up.

It is indeed possible for the boot CPU to be a nohz_full CPU and as
you can see, it's only half-working. This is so ever since:

    08ae95f4fd3b (nohz_full: Allow the boot CPU to be nohz_full)

I wish I had nacked it before it got merged, especially as the changelog mentions
that the user could have solved this with modifying its setup... I would love
to revert that now but I don't know if anyone uses this and have it working
by chance somewhere... Should we continue to support a broken feature? Can we
break user ABI if it's already half-broken?

Anyway so during boot it's possible to have an empty
housekeeping_mask(HK_TYPE_TIMER) & cpu_online_mask. After boot though (provided
any CPU from the housekeeping_mask(HK_TYPE_TIMER) has actually booted, which
isn't even guaranteed if maxcpus= is passed...) the first online housekeeping can't
go down like you spotted.

Thanks.