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

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Message-ID: <aXdxhfVLqyv-2SDX@willie-the-truck>
Date: Mon, 26 Jan 2026 13:52:05 +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

On Sun, Jan 25, 2026 at 11:45:36PM +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 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@...nel.org>
> Reviewed-by: Waiman Long <longman@...hat.com>
> ---
>  Documentation/arch/arm64/asymmetric-32bit.rst | 12 ++++++++----
>  arch/arm64/kernel/cpufeature.c                |  6 +++---
>  2 files changed, 11 insertions(+), 7 deletions(-)

Thanks, Frederic, this looks much better to me. I also chucked my old
tests at it and it looked good, so:

Acked-by: Will Deacon <will@...nel.org>
Tested-by: Will Deacon <will@...nel.org>

Will