Re: [PATCH 29/33] sched/arm64: Move fallback task cpumask to HK_TYPE_DOMAIN

[Waiman Long <llong@redhat.com>]

On 12/24/25 8:45 AM, Frederic Weisbecker wrote:
> When none of the allowed CPUs of a task are online, it gets migrated
> to the fallback cpumask which is all the non nohz_full CPUs.
>
> However just like nohz_full CPUs, domain isolated CPUs don't want to be
> disturbed by tasks that have lost their CPU affinities.
>
> And since nohz_full rely on domain isolation to work correctly, the
> housekeeping mask of domain isolated CPUs should always be a superset of
> the housekeeping mask of nohz_full CPUs (there can be CPUs that are
> domain isolated but not nohz_full, OTOH there shouldn't be nohz_full
> CPUs that are not domain isolated):
>
> 	HK_TYPE_DOMAIN | HK_TYPE_KERNEL_NOISE == HK_TYPE_DOMAIN
>
> Therefore use HK_TYPE_DOMAIN as the appropriate fallback target for
> tasks and since this cpumask can be modified at runtime, make sure
> that 32 bits support CPUs on ARM64 mismatched systems are not isolated
> by cpusets.
>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
>   arch/arm64/kernel/cpufeature.c | 18 +++++++++++++++---
>   include/linux/cpu.h            |  4 ++++
>   kernel/cgroup/cpuset.c         | 17 ++++++++++++++---
>   3 files changed, 33 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index c840a93b9ef9..70b0e45e299a 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1656,6 +1656,18 @@ has_cpuid_feature(const struct arm64_cpu_capabilities *entry, int scope)
>   	return feature_matches(val, entry);
>   }
>   
> +/*
> + * 32 bits support CPUs can't be isolated because tasks may be
> + * arbitrarily affine to them, defeating the purpose of isolation.
> + */
> +bool arch_isolated_cpus_can_update(struct cpumask *new_cpus)
> +{
> +	if (static_branch_unlikely(&arm64_mismatched_32bit_el0))
> +		return !cpumask_intersects(cpu_32bit_el0_mask, new_cpus);
> +	else
> +		return true;
> +}
> +
>   const struct cpumask *system_32bit_el0_cpumask(void)
>   {
>   	if (!system_supports_32bit_el0())
> @@ -1669,7 +1681,7 @@ const struct cpumask *system_32bit_el0_cpumask(void)
>   
>   const struct cpumask *task_cpu_fallback_mask(struct task_struct *p)
>   {
> -	return __task_cpu_possible_mask(p, housekeeping_cpumask(HK_TYPE_TICK));
> +	return __task_cpu_possible_mask(p, housekeeping_cpumask(HK_TYPE_DOMAIN));
>   }
>   
>   static int __init parse_32bit_el0_param(char *str)
> @@ -3987,8 +3999,8 @@ static int enable_mismatched_32bit_el0(unsigned int cpu)
>   	bool cpu_32bit = false;
>   
>   	if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0)) {
> -		if (!housekeeping_cpu(cpu, HK_TYPE_TICK))
> -			pr_info("Treating adaptive-ticks CPU %u as 64-bit only\n", cpu);
> +		if (!housekeeping_cpu(cpu, HK_TYPE_DOMAIN))
> +			pr_info("Treating domain isolated CPU %u as 64-bit only\n", cpu);
>   		else
>   			cpu_32bit = true;
>   	}
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index 487b3bf2e1ea..0b48af25ab5c 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -229,4 +229,8 @@ static inline bool cpu_attack_vector_mitigated(enum cpu_attack_vectors v)
>   #define smt_mitigations SMT_MITIGATIONS_OFF
>   #endif
>   
> +struct cpumask;
> +
> +bool arch_isolated_cpus_can_update(struct cpumask *new_cpus);
> +
>   #endif /* _LINUX_CPU_H_ */
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index cd6119c02beb..1cc83a3c25f6 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1408,14 +1408,22 @@ static void partition_xcpus_del(int old_prs, struct cpuset *parent,
>   	cpumask_or(parent->effective_cpus, parent->effective_cpus, xcpus);
>   }
>   
> +bool __weak arch_isolated_cpus_can_update(struct cpumask *new_cpus)
> +{
> +	return true;
> +}
> +
>   /*
> - * isolated_cpus_can_update - check for isolated & nohz_full conflicts
> + * isolated_cpus_can_update - check for conflicts against housekeeping and
> + *                            CPUs capabilities.
>    * @add_cpus: cpu mask for cpus that are going to be isolated
>    * @del_cpus: cpu mask for cpus that are no longer isolated, can be NULL
>    * Return: false if there is conflict, true otherwise
>    *
> - * If nohz_full is enabled and we have isolated CPUs, their combination must
> - * still leave housekeeping CPUs.
> + * Check for conflicts:
> + * - If nohz_full is enabled and there are isolated CPUs, their combination must
> + *   still leave housekeeping CPUs.
> + * - Architecture has CPU capabilities incompatible with being isolated
>    *
>    * TBD: Should consider merging this function into
>    *      prstate_housekeeping_conflict().
> @@ -1426,6 +1434,9 @@ static bool isolated_cpus_can_update(struct cpumask *add_cpus,
>   	cpumask_var_t full_hk_cpus;
>   	int res = true;
>   
> +	if (!arch_isolated_cpus_can_update(add_cpus))
> +		return false;
> +
>   	if (!housekeeping_enabled(HK_TYPE_KERNEL_NOISE))
>   		return true;
>   
Reviewed-by: Waiman Long <longman@redhat.com>