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Message-ID: <aNJrp_IDiDXEx6N4@2a01cb069018a810e4ede1071806178f.ipv6.abo.wanadoo.fr>
Date: Tue, 23 Sep 2025 11:43:03 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Waiman Long <llong@...hat.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
Catalin Marinas <catalin.marinas@....com>,
Marco Crivellari <marco.crivellari@...e.com>,
Michal Hocko <mhocko@...e.com>,
Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>, Will Deacon <will@...nel.org>,
linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 27/33] sched/arm64: Move fallback task cpumask to
HK_TYPE_DOMAIN
Le Tue, Sep 02, 2025 at 12:43:16PM -0400, Waiman Long a écrit :
>
> On 8/29/25 11:48 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 is always a subset of the
> > housekeeping mask of nohz_full CPUs (there can be CPUs that are domain
> > isolated but not nohz_full, OTOH there can'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.
> >
> > CC: linux-arm-kernel@...ts.infradead.org
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > ---
> > arch/arm64/kernel/cpufeature.c | 18 ++++++++++++---
> > include/linux/cpu.h | 4 ++++
> > kernel/cgroup/cpuset.c | 40 +++++++++++++++++++++++-----------
> > 3 files changed, 46 insertions(+), 16 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 9ad065f15f1d..38046489d2ea 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1653,6 +1653,18 @@ 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())
> > @@ -1666,7 +1678,7 @@ 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)
> > @@ -3963,8 +3975,8 @@ 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 b91b993f58ee..8bb239080534 100644
> > --- a/include/linux/cpu.h
> > +++ b/include/linux/cpu.h
> > @@ -228,4 +228,8 @@ 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 8260dd699fd8..cf99ea844c1d 100644
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -1352,33 +1352,47 @@ static bool partition_xcpus_del(int old_prs, struct cpuset *parent,
> > return isolcpus_updated;
> > }
> > +bool __weak arch_isolated_cpus_can_update(struct cpumask *new_cpus)
> > +{
> > + return true;
> > +}
> > +
> > /*
> > - * isolcpus_nohz_conflict - check for isolated & nohz_full conflicts
> > + * isolated_cpus_can_update - check for conflicts against housekeeping and
> > + * CPUs capabilities.
> > * @new_cpus: cpu mask for cpus that are going to be isolated
> > - * Return: true if there is conflict, false otherwise
> > + * Return: true if there no conflict, false 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
> > */
> > -static bool isolcpus_nohz_conflict(struct cpumask *new_cpus)
> > +static bool isolated_cpus_can_update(struct cpumask *new_cpus)
> > {
> > cpumask_var_t full_hk_cpus;
> > - int res = false;
> > + bool res;
> > +
> > + if (!arch_isolated_cpus_can_update(new_cpus))
> > + return false;
> > if (!housekeeping_enabled(HK_TYPE_KERNEL_NOISE))
> > - return false;
> > + return true;
> > if (!alloc_cpumask_var(&full_hk_cpus, GFP_KERNEL))
> > - return true;
> > + return false;
> > +
> > + res = true;
> > cpumask_and(full_hk_cpus, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE),
> > housekeeping_cpumask(HK_TYPE_DOMAIN));
> > cpumask_andnot(full_hk_cpus, full_hk_cpus, isolated_cpus);
> > cpumask_and(full_hk_cpus, full_hk_cpus, cpu_online_mask);
>
> We should construct the new cpumask by adding new CPUs and removing old ones
> from the existing isolated_cpus and pass it to
> arch_isolated_cpus_can_update() for the checking to be correct.
As for arch_isolated_cpus_can_update(), arm64 only cares about the new
CPUs that are going to be isolated. But perhaps you're referring to the check
above that excludes isolated_cpus while we don't care about the old set?
Thanks.
> Cheers,
> Longman
>
--
Frederic Weisbecker
SUSE Labs
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