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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Maxime Coquelin , Alexandre Torgue , Yury Norov , Rasmus Villemoes , Andrew Morton , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , Thomas Gleixner , Jan Kiszka , netdev@vger.kernel.org, linux-stm32@st-md-mailman.stormreply.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev Subject: Re: [PATCH RFC 1/3] cpumask: Honor irq_default_affinity in cpumask_local_spread() Message-ID: References: <20260819-flo-net-7-2-make-stmmac-default-affinity-aware-v1-0-3f79a99cadaf@siemens.com> <20260819-flo-net-7-2-make-stmmac-default-affinity-aware-v1-1-3f79a99cadaf@siemens.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260819-flo-net-7-2-make-stmmac-default-affinity-aware-v1-1-3f79a99cadaf@siemens.com> X-ClientProxiedBy: BN9PR03CA0567.namprd03.prod.outlook.com (2603:10b6:408:138::32) To LV3PR12MB9356.namprd12.prod.outlook.com (2603:10b6:408:20c::21) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: LV3PR12MB9356:EE_|DM4PR12MB6399:EE_ X-MS-Office365-Filtering-Correlation-Id: fa87580b-3fb1-43bf-12e2-08defe210d58 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|7416014|376014|366016|1800799024|6133799003|3023799007|56012099006|10067099003|4143699003|11063799006|22082099003|18002099003; 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For > realtime (PREEMPT_RT) and other cpu-isolating workloads the old > implementation was triggering an IRQ placement problem. IRQs were > targeting CPUs that were isolated for those sensitive workloads. > > Userland will tell the kernel about the desired IRQ configuration > for new interrupts by writing a proper cpumask to > /proc/irq/default_smp_affinity. This cpu mask has to be honored to > avoid IRQ noise on isolated cores. > > The default for irq_default_affinity is "all CPUs". So all CPUs will > be taken into account for spreading when userland did not configure > something special. > --- > lib/cpumask.c | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > diff --git a/lib/cpumask.c b/lib/cpumask.c > index 5adb9874fbd0f5a42ea8cd9e6c3729a70599781f..73e7b60a9201174f83df6067effbe7d1889dcdb9 100644 > --- a/lib/cpumask.c > +++ b/lib/cpumask.c > @@ -6,6 +6,7 @@ > #include > #include > #include > +#include > > /* These are not inline because of header tangles. */ > #ifdef CONFIG_CPUMASK_OFFSTACK > @@ -81,8 +82,9 @@ void __init free_bootmem_cpumask_var(cpumask_var_t mask) > * @i: index number > * @node: local numa_node > * > - * Return: online CPU according to a numa aware policy; local cpus are returned > - * first, followed by non-local ones, then it wraps around. > + * Return: online CPU according to the default IRQ affinity and a numa aware > + * policy; local cpus are returned first, followed by non-local ones, then it > + * wraps around. > * > * For those who wants to enumerate all CPUs based on their NUMA distances, > * i.e. call this function in a loop, like: > @@ -110,9 +112,9 @@ unsigned int cpumask_local_spread(unsigned int i, int node) Please don't touch this function. There's ~40 users, and we don't want to inspect every caller for their intention. Looking at ionic_get_preferred_cpu(), the cpumask_local_spread() is used there as a fallback for the affinity IRQ search: static int ionic_get_preferred_cpu(struct ionic *ionic, struct ionic_intr_info *intr) { int cpu; cpu = cpumask_first_and(*intr->affinity_mask, cpu_online_mask); if (cpu >= nr_cpu_ids) cpu = cpumask_local_spread(0, dev_to_node(ionic->dev)); return cpu; } Your change may affect the logic, seemingly. Instead, please create something like: unsigned int cpumask_spread(struct cpumask cpus, unsigned int i, int node); Thanks, Yury > unsigned int cpu; > > /* Wrap: we always want a cpu. */ > - i %= num_online_cpus(); > + i %= cpumask_weight(irq_default_affinity); > > - cpu = sched_numa_find_nth_cpu(cpu_online_mask, i, node); > + cpu = sched_numa_find_nth_cpu(irq_default_affinity, i, node); > > WARN_ON(cpu >= nr_cpu_ids); > return cpu; > > -- > 2.55.0