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Date: Mon, 15 Apr 2024 23:37:49 +0200
From: Frederic Weisbecker <frederic@kernel.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Nicholas Piggin <npiggin@gmail.com>, Leonardo Bras <leobras@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Tejun Heo <tj@kernel.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	linux-kernel@vger.kernel.org, Junyao Zhao <junzhao@redhat.com>,
	Chris von Recklinghausen <crecklin@redhat.com>
Subject: Re: [PATCH] sched/isolation: fix boot crash when the boot CPU is
 nohz_full
Message-ID: <Zh2eLUFm3LQvYVtF@pavilion.home>
References: <20240130010046.2730139-2-leobras@redhat.com>
 <20240402105847.GA24832@redhat.com>
 <20240411143905.GA19288@redhat.com>
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In-Reply-To: <20240411143905.GA19288@redhat.com>

Le Thu, Apr 11, 2024 at 04:39:05PM +0200, Oleg Nesterov a ?crit :
> Documentation/timers/no_hz.rst states that the "nohz_full=" mask must not
> include the boot CPU, this is no longer true after the commit 08ae95f4fd3b
> ("nohz_full: Allow the boot CPU to be nohz_full").
> 
> However after another commit aae17ebb53cd ("workqueue: Avoid using isolated
> cpus' timers on queue_delayed_work") the kernel will crash at boot time in
> this case; housekeeping_any_cpu() returns an invalid cpu nr until smp_init()
> paths bring the 1st housekeeping CPU up.
> 
> Change housekeeping_any_cpu() to check the result of cpumask_any_and() and
> return smp_processor_id() in this case. Yes, this is just the simple and
> backportable workaround which fixes the symptom, but smp_processor_id() at
> boot time should be safe at least for type == HK_TYPE_TIMER, this more or
> less matches the tick_do_timer_boot_cpu logic.
> 
> We should not worry about cpu_down(); tick_nohz_cpu_down() will not allow
> to offline tick_do_timer_cpu (the 1st online housekeeping CPU).
> 
> Fixes: aae17ebb53cd ("workqueue: Avoid using isolated cpus' timers on queue_delayed_work")
> Reported-by: Chris von Recklinghausen <crecklin@redhat.com>
> Closes: https://lore.kernel.org/all/20240402105847.GA24832@redhat.com/
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: Frederic Weisbecker <frederic@kernel.org>