linux-kernel - Re: [PATCH] sched/isolation: fix boot crash when maxcpus

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Message-ID: <ZiE--7rz1vuEudAm@localhost.localdomain>
Date: Thu, 18 Apr 2024 17:40:43 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Nicholas Piggin <npiggin@...il.com>, Leonardo Bras <leobras@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	linux-kernel@...r.kernel.org, Junyao Zhao <junzhao@...hat.com>,
	Chris von Recklinghausen <crecklin@...hat.com>
Subject: Re: [PATCH] sched/isolation: fix boot crash when maxcpus <
 first-housekeeping-cpu

Le Sat, Apr 13, 2024 at 04:17:46PM +0200, Oleg Nesterov a écrit :
> 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@...hat.com>

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