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

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

Hi Yury,

On 11/12/26 2:07 AM, Yury Norov wrote:
> On Tue, Nov 10, 2026 at 04:15:31PM +1300, Maxim Kuvyrkov wrote:
>>> On Nov 2, 2026, at 08:21, Yury Norov <ynorov@nvidia.com> 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.
> 
> Hi Maxim,
> 
> Thank you for the feedback.
> 
> I agree, this trick with pinning a workload to an non-preferred CPU
> looks promising for benchmarking purposes, and probably for more.
> 

Sorry, i don't follow.

Did you mean pin to preferred CPUs? Why would anyone want to run their
workload only on non-preferred CPUs? By definition, they are not meant to
be used as much as possible. and It is benchmarking.

> I'm going to split this series to the cleanup and follow-up parts.
> In the follow-up, I'll focus on this writable interface. I'll keep
> you in CC there.
> 
> Thanks,
> Yury