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

[Yury Norov <ynorov@nvidia.com>]

On Tue, Nov 10, 2026 at 02:45:58PM +0530, Shrikanth Hegde wrote:
> Hi,
> 
> On 11/10/26 8:45 AM, 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.
> > 
> 
> 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)

How that? I tried, and offlining destroys the affinity, as it should:

  # taskset -c 3 sh -c 'while :; do :; done' &
  [1] 380
  # taskset -pc "$pid"
  pid 380's current affinity list: 3
  # echo 0 >/sys/devices/system/cpu/cpu3/online
  [  115.767634] process 380 (sh) no longer affine to cpu3
  [  115.772881] smpboot: CPU 3 is now offline
  taskset -pc 380
  pid 380's current affinity list: 0-2,4-7

Did you mean something else, maybe?