Re: [PATCH v7 7/7] timers: Exclude isolated cpus from timer migration

[Frederic Weisbecker <frederic@kernel.org>]

Le Thu, Jun 26, 2025 at 01:49:00PM +0200, Gabriele Monaco a écrit :
> The timer migration mechanism allows active CPUs to pull timers from
> idle ones to improve the overall idle time. This is however undesired
> when CPU intensive workloads run on isolated cores, as the algorithm
> would move the timers from housekeeping to isolated cores, negatively
> affecting the isolation.
> 
> Exclude isolated cores from the timer migration algorithm, extend the
> concept of unavailable cores, currently used for offline ones, to
> isolated ones:
> * A core is unavailable if isolated or offline;
> * A core is available if isolated and offline;

If not isolated and online ?

> +static int __init tmigr_init_isolation(void)
> +{
> +	cpumask_var_t cpumask;
> +
> +	if (!housekeeping_enabled(HK_TYPE_DOMAIN))
> +		return 0;
> +	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
> +		return -ENOMEM;
> +	cpumask_andnot(cpumask, tmigr_available_cpumask,
> +		       housekeeping_cpumask(HK_TYPE_DOMAIN));
> +	cpumask_and(cpumask, cpumask, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
> +	on_each_cpu_mask(cpumask, tmigr_cpu_isolate, NULL, 1);
> +
> +	tmigr_exclude_isolated = true;

This should be set unconditionally, even if
!housekeeping_enabled(HK_TYPE_DOMAIN).

Otherwise future cpuset isolation followed by cpuset de-isolation
won't make the CPUs back available.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs