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Date: Sat, 6 Apr 2024 00:03:50 +0200
From: Frederic Weisbecker <frederic@kernel.org>
To: Tejun Heo <tj@kernel.org>, Nick Piggin <npiggin@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>, 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>
Subject: Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on
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Le Fri, Apr 05, 2024 at 05:38:54AM -1000, Tejun Heo a ?crit :
> > > > > 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) {
> 
> Ensuring the boot CPU always be a housekeeping CPU makes sense to me but I'm
> not very familiar with the housekeeping code. Frederic, what do you think?

I vote for reverting:

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

And if that's not possible because there are actual users of it, then
the best we can do is to make housekeeping_any_cpu() return smp_processor_id()
if the housekeeping_mask() is empty. Or rather if the housekeeping_mask is
empty, return cpumask_of(smp_processor_id()).

Of course that's merely just a bandaid.

Thanks.