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Date: Sun, 28 Apr 2024 15:16:45 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org,
	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: <20240428131645.GA20436@redhat.com>
References: <20240413141746.GA10008@redhat.com>
 <171398910207.10875.4426725644764756607.tip-bot2@tip-bot2>
 <Zi4FHsc51wNhdSW4@gmail.com>
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On 04/28, Ingo Molnar wrote:
>
> * 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?

Or the user passes the empty "nohz_full=" mask on a multi-CPU machine.

In both cases (before this patch) housekeeping_setup() calls
tick_nohz_full_setup(non_housekeeping_mask) which sets
tick_nohz_full_running = true even if tick_nohz_full_mask is empty.

This doesn't look right to me and triggers the "this should not happen"
warning in tick_sched_do_timer().

But let me repeat, I know nothing about nohz/etc.

Oleg.