linux-kernel - Re: [PATCH v6 6/6] timers: Exclude isolated cpus from timer migation

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Message-ID: <aFq7rqbHugtiWF6Z@localhost.localdomain>
Date: Tue, 24 Jun 2025 16:52:30 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v6 6/6] timers: Exclude isolated cpus from timer migation

Le Tue, Jun 24, 2025 at 04:06:41PM +0200, Gabriele Monaco a écrit :
> Right, but as far as I understood, the first call to 
> tmigr_set_cpu_available() happens after the isolcpus parameter has been
> parsed so we know at least cpu0 is going to be isolated.
> 
> On my machine it works reliably this way. I'm a bit lost in the init
> code but seeing housekeeping_init() before rcu_init(), which in turn
> should be required for some RCU-related early_initcalls, makes me
> believe this order is guaranteed to be respected.
> Or am I missing something?

Right I think you need to keep those checks because if CPU 0 is isolcpus
and CPU 5 is nohz_full, CPU 0 will become later the timekeeper and must stay
in the tmigr hierarchy.

OTOH if CPU 0 is isolcpus and there is no nohz_full CPUs, then CPU 0 doesn't
want to go to the hierarchy.

cpuset isolated partitions are different because they issue SMP calls whereas
isolcpus is defined on boot.

An alternative for isolcpus could be to make a late initcall and do the smp
calls from there just like is done for cpusets.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs