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Message-ID: <19b001a4-0696-4efd-aaa3-902ffa42ff9e@redhat.com>
Date: Fri, 26 Dec 2025 18:46:02 -0500
From: Waiman Long <llong@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>,
LKML <linux-kernel@...r.kernel.org>
Cc: Michal Koutný <mkoutny@...e.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Bjorn Helgaas <bhelgaas@...gle.com>,
Catalin Marinas <catalin.marinas@....com>,
Chen Ridong <chenridong@...wei.com>, Danilo Krummrich <dakr@...nel.org>,
"David S . Miller" <davem@...emloft.net>, Eric Dumazet
<edumazet@...gle.com>, Gabriele Monaco <gmonaco@...hat.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Ingo Molnar <mingo@...hat.com>, Jakub Kicinski <kuba@...nel.org>,
Jens Axboe <axboe@...nel.dk>, Johannes Weiner <hannes@...xchg.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Marco Crivellari <marco.crivellari@...e.com>, Michal Hocko
<mhocko@...e.com>, Muchun Song <muchun.song@...ux.dev>,
Paolo Abeni <pabeni@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
Phil Auld <pauld@...hat.com>, "Rafael J . Wysocki" <rafael@...nel.org>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Shakeel Butt <shakeel.butt@...ux.dev>, Simon Horman <horms@...nel.org>,
Tejun Heo <tj@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
Vlastimil Babka <vbabka@...e.cz>, Will Deacon <will@...nel.org>,
cgroups@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
linux-block@...r.kernel.org, linux-mm@...ck.org, linux-pci@...r.kernel.org,
netdev@...r.kernel.org
Subject: Re: [PATCH 29/33] sched/arm64: Move fallback task cpumask to
HK_TYPE_DOMAIN
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@...nel.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@...hat.com>
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