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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date: Sun, 28 Apr 2024 10:13:18 +0200
From: Ingo Molnar <mingo@kernel.org>
To: linux-kernel@vger.kernel.org
Cc: linux-tip-commits@vger.kernel.org, Oleg Nesterov <oleg@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>, Phil Auld <pauld@redhat.com>,
	Frederic Weisbecker <frederic@kernel.org>, x86@kernel.org
Subject: Re: [tip: sched/urgent] sched/isolation: Fix boot crash when maxcpus
 < first housekeeping CPU
Message-ID: <Zi4FHsc51wNhdSW4@gmail.com>
References: <20240413141746.GA10008@redhat.com>
 <171398910207.10875.4426725644764756607.tip-bot2@tip-bot2>
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* tip-bot2 for Oleg Nesterov <tip-bot2@linutronix.de> wrote:

> 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 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? 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.

So arguably the user meant NOHZ_FULL to be turned off - but it is de-facto 
already turned off by the fact that there's only a single CPU available, 
right?

Thanks,

	Ingo