Re: [PATCH 0/6] sched: Expose writable interface for preferred CPUs mask

[Shrikanth Hegde]

Hi,

On 11/10/26 8:45 AM, Maxim Kuvyrkov wrote:
> > On Nov 2, 2026, at 08:21, Yury Norov <ynorov@xxxxxxxxxx> wrote:
> >
> > The preferred CPU mask currently lets the steal governor reduce vCPU
> > demand when physical CPUs are contended. This series adds manual control,
> > opening more use cases.
> >
> > For example, setting CPUs aside for selected fair tasks at runtime while
> > steering other fair tasks toward the remaining CPUs.
>
> Hi Yury,
>
> I've been trying to do this with crutches in user-space for a long time.  Thanks for implementing this in the kernel, and with a convenient user interface.
>
> > A user can mark a CPU non-preferred and pin a workload to it. Fair tasks
> > whose affinity includes preferred CPUs are steered away, while tasks
> > whose allowed CPUs contain no preferred CPU can still run there. The CPU
> > stays online, task affinity is preserved, and the selection can be
> > reversed without rebooting:
> >
> >     # rmmod steal_governor
> >     # echo 0 > /sys/devices/system/cpu/cpu3/preferred
> >     # taskset -c 3 ./my_workload
> >     # echo 1 > /sys/devices/system/cpu/cpu3/preferred
>
> My use-case (being a compiler optimization engineer) is to pin benchmarks to specific cores, and stop the system from using those cores even when the cores are idle.  This prevents L1/L2 cache pollution and makes performance numbers more stable.

Do you change it when running workload based on any signal/event? or you set it before starting it?

If it is set before starting, you might as well offline the CPUs to achieve the same. I assume you are
managing the preferred CPUs manually with write interface. You can do the same for online state.
(Though it takes a longer)

> Thanks!
>
> --
> Maxim Kuvyrkov
> Garden City Compilers