linux-kernel - Re: [PATCH v14 7/7] timers: Exclude isolated cpus from timer migration

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Message-ID: <446f97f9663c1bda011d4171d17f1470d21fb7df.camel@redhat.com>
Date: Wed, 12 Nov 2025 17:18:26 +0100
From: Gabriele Monaco <gmonaco@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: linux-kernel@...r.kernel.org, Anna-Maria Behnsen
 <anna-maria@...utronix.de>,  Thomas Gleixner <tglx@...utronix.de>, Waiman
 Long <llong@...hat.com>, "John B . Wyatt IV" <jwyatt@...hat.com>,  "John B
 . Wyatt IV" <sageofredondo@...il.com>
Subject: Re: [PATCH v14 7/7] timers: Exclude isolated cpus from timer
 migration

On Wed, 2025-11-12 at 17:04 +0100, Frederic Weisbecker wrote:
> Le Tue, Nov 04, 2025 at 11:47:39AM +0100, Gabriele Monaco a écrit :
> 
> 
> This duplicates a lot tmigr_isolated_exclude_cpumask().
> Would this work?
> 
> static int __init tmigr_init_isolation(void)
> {
> 	cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
> 
> 	static_branch_enable(&tmigr_exclude_isolated);
> 
> 	if (!housekeeping_enabled(HK_TYPE_DOMAIN))
> 		return 0;
> 	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
> 		return -ENOMEM;
> 
> 	cpumask_andnot(cpumask, cpu_possible_mask,
> housekeeping_cpumask(HK_TYPE_DOMAIN));
> 
> 	return tmigr_isolated_exclude_cpumask(cpumask);
> }
> 
> If so please add my Reviewed-by to the next version.

Good point, it should work, I'm going to test it tomorrow.

What comes to my mind is that tmigr_isolated_exclude_cpumask expects the hotplug
lock to be held, so I should probably just hold it before calling it:

static int __init tmigr_init_isolation(void)
{
	cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;

	static_branch_enable(&tmigr_exclude_isolated);

	if (!housekeeping_enabled(HK_TYPE_DOMAIN))
		return 0;
	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
		return -ENOMEM;

	cpumask_andnot(cpumask, cpu_possible_mask,
housekeeping_cpumask(HK_TYPE_DOMAIN));
+	guard(cpus_read_lock)();
	return tmigr_isolated_exclude_cpumask(cpumask);
}

Thanks,
Gabriele