Re: 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 <frederic@kernel.org>]

Le Sun, Apr 07, 2024 at 03:09:14PM +0200, Oleg Nesterov a écrit :
> On 04/05, Frederic Weisbecker wrote:
> >
> > +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)
> 
> Thanks... So this is intentional. I was confused by
> 
> 	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.
> 
> from Documentation/timers/no_hz.rst
> 
> > 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?
> 
> Well, the changelog says
> 
>     nohz_full has been trialed at a large supercomputer site and found to
>     significantly reduce jitter. In order to deploy it in production, they
>     need CPU0 to be nohz_full
> 
> so I guess this feature has users.
> 
> But after the commit aae17ebb53cd3da ("workqueue: Avoid using isolated cpus'
> timers on queue_delayed_work") the kernel will crash at boot time if the boot
> CPU is nohz_full.

Right but there are many possible calls to housekeeping on boot before the
first housekeeper becomes online.

Thanks.