Re: [PATCH v7 2/7] timers: Add the available mask in timer migration

[Frederic Weisbecker <frederic@kernel.org>]

Le Thu, Jun 26, 2025 at 01:48:55PM +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@redhat.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 2c2c8810b8137..2ec003c2bbbea 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_clear_cpu_available(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);

This is under the tmc lock.

>  
>  	/*
>  	 * CPU has to handle the local events on his own, when on the way to
> @@ -1459,7 +1463,7 @@ static int tmigr_clear_cpu_available(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);

But this is not.

We should make it clear what synchronizes the accesses to
tmigr_available_cpumask.

ie: we expect that either the cpu_hotplug_lock is write-held, or
it's read-held and cpuset_mutex is held.

But tmc->lock is not helpful. So the cpumask should be outside that
lock to avoid confusion.

Thanks.

>  		work_on_cpu(migrator, tmigr_trigger_active, NULL);
>  	}
>  
> @@ -1480,6 +1484,7 @@ static int tmigr_set_cpu_available(unsigned int cpu)
>  	if (!tmc->idle)
>  		__tmigr_cpu_activate(tmc);
>  	tmc->available = true;
> +	cpumask_set_cpu(cpu, tmigr_available_cpumask);
>  	raw_spin_unlock_irq(&tmc->lock);
>  	return 0;
>  }
> @@ -1801,6 +1806,11 @@ static int __init tmigr_init(void)
>  	if (ncpus == 1)
>  		return 0;
>  
> +	if (!zalloc_cpumask_var(&tmigr_available_cpumask, GFP_KERNEL)) {
> +		ret = -ENOMEM;
> +		goto err;
> +	}
> +
>  	/*
>  	 * Calculate the required hierarchy levels. Unfortunately there is no
>  	 * reliable information available, unless all possible CPUs have been
> -- 
> 2.49.0
> 

-- 
Frederic Weisbecker
SUSE Labs