linux-kernel - Re: [PATCH v11 8/8] timers: Exclude isolated cpus from timer migration

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Message-ID: <aLWUkpKgFFVr_TEx@localhost.localdomain>
Date: Mon, 1 Sep 2025 14:41:54 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: linux-kernel@...r.kernel.org,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v11 8/8] timers: Exclude isolated cpus from timer
 migration

Le Fri, Aug 08, 2025 at 06:01:42PM +0200, Gabriele Monaco a écrit :
>  /*
>   * NOHZ can only be enabled after clocksource_done_booting(). Don't
>   * bother trashing the cache in the tree before.
>   */
>  static int __init tmigr_late_init(void)
>  {
> -	return cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
> -				 tmigr_set_cpu_available, tmigr_clear_cpu_available);
> +	int cpu, ret;
> +
> +	ret = cpuhp_setup_state(CPUHP_AP_TMIGR_ONLINE, "tmigr:online",
> +				tmigr_set_cpu_available, tmigr_clear_cpu_available);
> +	if (ret)
> +		return ret;
> +	/*
> +	 * The tick CPU may not be marked as available in the above call, this
> +	 * can occur only at boot as hotplug handlers are not called on the
> +	 * tick CPU. Force it enabled here.
> +	 */
> +	for_each_possible_cpu(cpu) {
> +		if (!tick_nohz_cpu_hotpluggable(cpu)) {
> +			ret = smp_call_function_single(
> +				cpu, tmigr_cpu_unisolate_force, NULL, 1);
> +			break;
> +		}
> +	}

Why not evaluate tick_nohz_cpu_hotpluggable() from tmigr_clear_cpu_available()
instead of this force IPI?

But if I understand correctly, this will be handled by cpuset, right?

Thanks.

-- 
Frederic Weisbecker
SUSE Labs