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Date: Fri, 5 Apr 2024 16:04:49 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Tejun Heo <tj@kernel.org>
Cc: Leonardo Bras <leobras@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	linux-kernel@vger.kernel.org, Junyao Zhao <junzhao@redhat.com>,
	Chris von Recklinghausen <crecklin@redhat.com>,
	Frederic Weisbecker <frederic@kernel.org>
Subject: Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on
 queue_delayed_work
Message-ID: <20240405140449.GB22839@redhat.com>
References: <20240130010046.2730139-2-leobras@redhat.com>
 <20240402105847.GA24832@redhat.com>
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 <20240403203814.GD31764@redhat.com>
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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?

Oleg.

> > > So it seems that we should fix housekeeping_setup() ? see the patch below.
> > >
> > > In any case the usage of cpu_present_mask doesn't look right to me.
> > >
> > > Oleg.
> > >
> > > --- a/kernel/sched/isolation.c
> > > +++ b/kernel/sched/isolation.c
> > > @@ -129,7 +154,7 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
> > >  	cpumask_andnot(housekeeping_staging,
> > >  		       cpu_possible_mask, non_housekeeping_mask);
> > >
> > > -	if (!cpumask_intersects(cpu_present_mask, housekeeping_staging)) {
> > > +	if (!cpumask_test_cpu(smp_processor_id(), housekeeping_staging)) {
> > >  		__cpumask_set_cpu(smp_processor_id(), housekeeping_staging);
> > >  		__cpumask_clear_cpu(smp_processor_id(), non_housekeeping_mask);
> > >  		if (!housekeeping.flags) {
> > >