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sched/arm64: Move fallback task cpumask to HK_TYPE_DOMAIN

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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 subset 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. Note that cpuset isolated partitions are not supported on those
systems and may result in undefined behaviour.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Waiman Long <longman@redhat.com>
Acked-by: Will Deacon <will@kernel.org>
Tested-by: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marco Crivellari <marco.crivellari@suse.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <longman@redhat.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
This commit is contained in:
Frederic Weisbecker 2025-07-24 23:38:48 +02:00
parent bf5b0c27fa
commit 79c0abaf06
2 changed files with 11 additions and 7 deletions
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12
Documentation/arch/arm64/asymmetric-32bit.rst
View File

@ -154,10 +154,14 @@ mode will return to host userspace with an ``exit_reason`` of
``KVM_EXIT_FAIL_ENTRY`` and will remain non-runnable until successfully
re-initialised by a subsequent ``KVM_ARM_VCPU_INIT`` operation.
NOHZ FULL
---------
SCHEDULER DOMAIN ISOLATION
--------------------------
To avoid perturbing an adaptive-ticks CPU (specified using
``nohz_full=``) when a 32-bit task is forcefully migrated, these CPUs
To avoid perturbing a boot-defined domain isolated CPU (specified using
``isolcpus=[domain]``) when a 32-bit task is forcefully migrated, these CPUs
are treated as 64-bit-only when support for asymmetric 32-bit systems
is enabled.
However as opposed to boot-defined domain isolation, runtime-defined domain
isolation using cpuset isolated partition is not advised on asymmetric
32-bit systems and will result in undefined behaviour.

6
arch/arm64/kernel/cpufeature.c
View File

@ -1669,7 +1669,7 @@ 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 +3987,8 @@ 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;
}