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

[Yury Norov <ynorov@nvidia.com>]

On Fri, Jul 10, 2026 at 10:30:43PM +0530, Shrikanth Hegde wrote:
> 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.

If I didn't enable preferred CPUs, I'll be pretty surprised having
them in my statistics.