Re: [PATCH v7 03/12] sysfs: Add preferred CPU file

[Yury Norov <ynorov@nvidia.com>]

On Fri, Jul 10, 2026 at 03:26:39AM +0530, Shrikanth Hegde wrote:
> Add "preferred" file in /sys/devices/system/cpu
> 
> This offers
> - User can quickly check which CPUs are marked as preferred at this
>   moment.
> - Userspace algorithms irqbalance could use this mask to send irq into
>   preferred CPUs.
> 
> For example:
> cat /sys/devices/system/cpu/online
> 0-719
> cat /sys/devices/system/cpu/preferred
> 0-599        <<< Implies 0-599 are preferred for workloads and 600-719
>                  should be avoided at this moment.
> 
> cat /sys/devices/system/cpu/preferred
> 0-719        <<< All CPUs are usable. There is no preference.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
>  Documentation/ABI/testing/sysfs-devices-system-cpu | 11 +++++++++++
>  drivers/base/cpu.c                                 |  8 ++++++++
>  2 files changed, 19 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
> index 82d10d556cc8..ac1dbb209cc7 100644
> --- a/Documentation/ABI/testing/sysfs-devices-system-cpu
> +++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
> @@ -806,3 +806,14 @@ Date:		Nov 2022
>  Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
>  Description:
>  		(RO) the list of CPUs that can be brought online.
> +
> +What:		/sys/devices/system/cpu/preferred
> +Date:		July 2026
> +Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
> +Description:
> +		(RO) the list of preferred CPUs at this moment.
> +		These are the only CPUs meant to be used at the moment.
> +		Using CPU outside of the list could lead to more
> +		contention of underlying physical CPU resource. Dynamically
> +		changes based on steal time. With CONFIG_PREFERRED_CPU=n it
> +		is same as active CPUs. See sched-arch.rst for more details.

This should mention that it's about paravirtualization.

> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> index 19d288a3c80c..4ac990efee7c 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -391,6 +391,13 @@ static int cpu_uevent(const struct device *dev, struct kobj_uevent_env *env)
>  }
>  #endif
>  
> +static ssize_t preferred_show(struct device *dev,
> +			      struct device_attribute *attr, char *buf)
> +{
> +	return sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(cpu_preferred_mask));
> +}
> +static DEVICE_ATTR_RO(preferred);
> +
>  const struct bus_type cpu_subsys = {
>  	.name = "cpu",
>  	.dev_name = "cpu",
> @@ -532,6 +539,7 @@ static struct attribute *cpu_root_attrs[] = {
>  #ifdef CONFIG_GENERIC_CPU_AUTOPROBE
>  	&dev_attr_modalias.attr,
>  #endif
> +	&dev_attr_preferred.attr,

 #ifdef CONFIG_PREFERRED_CPUS ?

>  	NULL
>  };
>  
> -- 
> 2.47.3