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

Le Thu, Jan 22, 2026 at 11:36:21AM +0000, Will Deacon a écrit :

On Thu, Jan 22, 2026 at 12:29:17PM +0100, Frederic Weisbecker wrote:

quoted

Le Thu, Jan 22, 2026 at 09:56:29AM +0000, Will Deacon a écrit :

quoted

On Wed, Jan 21, 2026 at 06:06:07PM +0100, Frederic Weisbecker wrote:

quoted

Le Tue, Jan 20, 2026 at 03:15:14PM +0000, Will Deacon a écrit :

quoted

Hi Frederic,

On Thu, Jan 01, 2026 at 11:13:54PM +0100, Frederic Weisbecker wrote:

quoted

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>
Reviewed-by: Waiman Long <longman@redhat.com>
---
 arch/arm64/kernel/cpufeature.c | 18 +++++++++++++++---
 include/linux/cpu.h            |  4 ++++
 kernel/cgroup/cpuset.c         | 17 ++++++++++++++---
 3 files changed, 33 insertions(+), 6 deletions(-)

tbh, I'd also be fine just saying that isolation isn't reliable on these
systems and then you don't need to add the extra arch hook.

Hmm, I think I heard about nohz_full usage on arm64 but I'm not sure.
And I usually expect isolcpus or cpuset isolated partitions to be even
more broadly used, it's lighter isolation with less constraints.

Anyway you're probably right that we could remove isolation support here
but I don't want to break any existing user.

fwiw, I think it's only some Android markets using the mismatched 32-bit
support and we're definitely not using nohz_full there.

Now that removal becomes appealing. And what about isolcpus= / isolated cpuset
which only consist in scheduler domain isolation? Probably not used by android
either.

Ok but is there a way to detect on early boot that the system has mismatched
32 bits support? Because I need to fail nohz_full= and isolcpus= boot parameters
early on top of this information without waiting for secondary CPUs boot.

Honestly, I'm not sure I'd bother trying to be smart here. Even if the
system has enabled support for mismatched 32-bit CPUs, things should
still work properly with nohz_full/isolcpus if all the tasks are 64-bit,
right?

In which case, I'd just document whatever weird behaviour you get if
somebody throws 32-bit tasks into the mix. Adding hooks to the generic
code for this use-case just seems like a waste, as they're not going to
be used in practice and it increases the maintenance burden.

Ok, how does this updated version look now?

---
From: Frederic Weisbecker <frederic@kernel.org>
Date: Thu, 24 Jul 2025 23:38:48 +0200
Subject: [PATCH] sched/arm64: Move fallback task cpumask to HK_TYPE_DOMAIN

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>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marco Crivellari <redacted>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <redacted>
Cc: Waiman Long <longman@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
---
 Documentation/arch/arm64/asymmetric-32bit.rst | 12 ++++++++----
 arch/arm64/kernel/cpufeature.c                |  6 +++---
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/Documentation/arch/arm64/asymmetric-32bit.rst b/Documentation/arch/arm64/asymmetric-32bit.rst
index 57b8d7476f71..fc0c350c5e00 100644
--- a/Documentation/arch/arm64/asymmetric-32bit.rst
+++ b/Documentation/arch/arm64/asymmetric-32bit.rst

@@ -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.

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index c840a93b9ef9..f0e66cb27d17 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c

@@ -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;
 	}

-- 
2.51.1