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Miller" , Eric Dumazet , Gabriele Monaco , Greg Kroah-Hartman , Ingo Molnar , Jakub Kicinski , Jens Axboe , Johannes Weiner , Lai Jiangshan , Marco Crivellari , Michal Hocko , Muchun Song , Paolo Abeni , Peter Zijlstra , Phil Auld , "Rafael J . Wysocki" , Roman Gushchin , Shakeel Butt , Simon Horman , Tejun Heo , Thomas Gleixner , Vlastimil Babka , Waiman Long , cgroups@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-block@vger.kernel.org, linux-mm@kvack.org, linux-pci@vger.kernel.org, netdev@vger.kernel.org Subject: Re: [PATCH 29/33] sched/arm64: Move fallback task cpumask to HK_TYPE_DOMAIN Message-ID: References: <20260101221359.22298-1-frederic@kernel.org> <20260101221359.22298-30-frederic@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Queue-Id: 5973E140002 X-Stat-Signature: j8zygj75tyjfrxz4zyctqsffebwxrj1d X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1769091954-262438 X-HE-Meta: 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 3vYMvb9X 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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: > > Le Thu, Jan 22, 2026 at 09:56:29AM +0000, Will Deacon a écrit : > > > On Wed, Jan 21, 2026 at 06:06:07PM +0100, Frederic Weisbecker wrote: > > > > Le Tue, Jan 20, 2026 at 03:15:14PM +0000, Will Deacon a écrit : > > > > > Hi Frederic, > > > > > > > > > > On Thu, Jan 01, 2026 at 11:13:54PM +0100, 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 > > > > > > Reviewed-by: Waiman Long > > > > > > --- > > > > > > 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 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 Cc: Catalin Marinas Cc: Marco Crivellari Cc: Michal Hocko Cc: Peter Zijlstra Cc: Tejun Heo Cc: Thomas Gleixner Cc: Waiman Long Cc: Will Deacon Cc: Catalin Marinas 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