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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-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260122_095644_462858_93708736 X-CRM114-Status: GOOD ( 29.35 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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. Will