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Date: Thu, 18 Apr 2024 17:40:43 +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 maxcpus <
 first-housekeeping-cpu
Message-ID: <ZiE--7rz1vuEudAm@localhost.localdomain>
References: <20240130010046.2730139-2-leobras@redhat.com>
 <20240402105847.GA24832@redhat.com>
 <20240411143905.GA19288@redhat.com>
 <20240413141746.GA10008@redhat.com>
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In-Reply-To: <20240413141746.GA10008@redhat.com>

Le Sat, Apr 13, 2024 at 04:17:46PM +0200, Oleg Nesterov a ?crit :
> housekeeping_setup() checks cpumask_intersects(present, online) to ensure
> that the kernel will have at least one housekeeping CPU after smp_init(),
> but this doesn't work if the maxcpus= kernel parameter limits the number
> of processors available after bootup.
> 
> For example, the kernel with "maxcpus=2 nohz_full=0-2" parameters crashes
> at boot time on my virtual machine with 4 CPUs.
> 
> Change housekeeping_setup() to use cpumask_first_and() and check that the
> returned cpu number is valid and less than setup_max_cpus.
> 
> Another corner case is "nohz_full=0" on a machine with a single CPU or
> with the maxcpus=1 kernel argument. In this case non_housekeeping_mask
> is empty and IIUC tick_nohz_full_setup() makes no sense. And indeed, the
> kernel hits the WARN_ON(tick_nohz_full_running) in tick_sched_do_timer().
> 
> And how should the kernel interpret the "nohz_full=" parameter? I think
> it should be silently ignored, but currently cpulist_parse() happily
> returns the empty cpumask and this leads to the same problem.
> 
> Change housekeeping_setup() to check cpumask_empty(non_housekeeping_mask)
> and do nothing in this case.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

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