linux-kernel - Re: [PATCH 29/33] sched/arm64: Move fallback task cpumask to HK_TYPE

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Message-ID: <aW-cAlJCtI5Qtify@willie-the-truck>
Date: Tue, 20 Jan 2026 15:15:14 +0000
From: Will Deacon <will@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	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>, Waiman Long <longman@...hat.com>,
	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

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 <frederic@...nel.org>
> Reviewed-by: Waiman Long <longman@...hat.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.

Whatever you prefer, but please can you update the text in
Documentation/arch/arm64/asymmetric-32bit.rst to cover the interaction
between the asymmetric stuff and cpu isolation?

Cheers,

Will