linux-kernel - Re: [PATCH v4 2/5] timers: Add the available mask in timer migration

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Message-ID: <aBozrJ0C6yzW7oB_@localhost.localdomain>
Date: Tue, 6 May 2025 18:07:08 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v4 2/5] timers: Add the available mask in timer migration

Le Tue, May 06, 2025 at 11:15:37AM +0200, Gabriele Monaco a écrit :
> Keep track of the CPUs available for timer migration in a cpumask. This
> prepares the ground to generalise the concept of unavailable CPUs.
> 
> Signed-off-by: Gabriele Monaco <gmonaco@...hat.com>
> ---
>  kernel/time/timer_migration.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
> index 7efd897c7959..25439f961ccf 100644
> --- a/kernel/time/timer_migration.c
> +++ b/kernel/time/timer_migration.c
> @@ -422,6 +422,9 @@ static unsigned int tmigr_crossnode_level __read_mostly;
>  
>  static DEFINE_PER_CPU(struct tmigr_cpu, tmigr_cpu);
>  
> +/* CPUs available for timer migration */
> +static cpumask_var_t tmigr_available_cpumask;
> +
>  #define TMIGR_NONE	0xFF
>  #define BIT_CNT		8
>  
> @@ -1449,6 +1452,7 @@ static int tmigr_cpu_unavailable(unsigned int cpu)
>  	raw_spin_lock_irq(&tmc->lock);
>  	tmc->available = false;
>  	WRITE_ONCE(tmc->wakeup, KTIME_MAX);
> +	cpumask_clear_cpu(cpu, tmigr_available_cpumask);
>  
>  	/*
>  	 * CPU has to handle the local events on his own, when on the way to
> @@ -1459,7 +1463,7 @@ static int tmigr_cpu_unavailable(unsigned int cpu)
>  	raw_spin_unlock_irq(&tmc->lock);
>  
>  	if (firstexp != KTIME_MAX) {
> -		migrator = cpumask_any_but(cpu_online_mask, cpu);
> +		migrator = cpumask_any(tmigr_available_cpumask);

Considering nohz_full CPUs should be still available.

I don't think there is anything ensuring that, in nohz_full mode,
there must be at least one housekeeping CPU that is not domain
isolated.

For example if we have two CPUs with CPU 0 being domain isolated
and CPU 1 being nohz_full, then there is no migrator to handle CPU 1's
global timers.

Thanks.

>  		work_on_cpu(migrator, tmigr_trigger_active, NULL);
>  	}

-- 
Frederic Weisbecker
SUSE Labs