Re: [PATCH v9 6/8] sched/isolation: Force housekeeping if isolcpus and nohz_full don't leave any

[Waiman Long]

On 7/30/25 9:11 AM, Gabriele Monaco wrote:
> Currently the user can set up isolcpus and nohz_full in such a way that
> leaves no housekeeping CPU (i.e. no CPU that is neither domain isolated
> nor nohz full). This can be a problem for other subsystems (e.g. the
> timer wheel imgration).
>
> Prevent this configuration by invalidating the last setting in case the
> union of isolcpus and nohz_full covers all CPUs.
>
> Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Signed-off-by: Gabriele Monaco <gmonaco@xxxxxxxxxx>
> ---
>   kernel/sched/isolation.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
>
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 93b038d48900..0019d941de68 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -165,6 +165,18 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
>   			}
>   		}
>   
> +		/* Check in combination with the previously set cpumask */
> +		type = find_first_bit(&housekeeping.flags, HK_TYPE_MAX);
> +		first_cpu = cpumask_first_and_and(cpu_present_mask,
> +						  housekeeping_staging,
> +						  housekeeping.cpumasks[type]);
> +		if (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) {
> +			pr_warn("Housekeeping: must include one present CPU neither "
> +				"in nohz_full= nor in isolcpus=, ignoring setting %s\n",
> +				str);
> +			goto free_housekeeping_staging;
> +		}
> +
>   		iter_flags = flags & ~housekeeping.flags;
>   
>   		for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)

I do have a question about this check. Currently isolcpus=domain is bit 
0, managed_irq is bit 1 and nohz_full is bit 2. If manage_irq come first 
followed by nohz_full and then isolcpus=domain. By the time, 
isolcpus=domain is being set, you are comparing its cpumask with that of 
manage_irq, not nohz_full.

Perhaps you can reuse the non_housekeeping_mask for doing the check, e.g.

        cpumask_and(non_housekeeping_mask, cpu_present_mask, 
housekeeping_staging);
        iter_flags = housekeeping.flags & ~flags;
        for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)
                cpumask_and(non_housekeeping_mask, 
non_housekeeping_mask, housekeeping.cpumasks[type]);
        if (cpumask_empty(non_housekeeping_mask)) {
                pr_warn(...

Regards,
Longman