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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Yury, thanks for taking a look.

On 7/10/26 9:11 PM, Yury Norov wrote:
> 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.

Ok. I will rephrase it.

> 
>> 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 ?

Not needed no? It will print active CPUs.

> 
>>   	NULL
>>   };
>>   
>> -- 
>> 2.47.3