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

[Peter Zijlstra]

On Tue, Nov 03, 2026 at 11:49:02AM -0500, Yury Norov wrote:
> On Mon, Nov 02, 2026 at 09:47:04AM +0100, Peter Zijlstra wrote:
> > On Sun, Nov 01, 2026 at 02:21:56PM -0500, Yury Norov 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.
> > 
> > Sceptical, I am.
> > 
> > > For example, setting CPUs aside for selected fair tasks at runtime while
> > > steering other fair tasks toward the remaining CPUs.
> > 
> > That's exactly the kind of nonsense we don't need and makes me want to
> > stop reading right here.
> 
> OK, I started with the wrong argument. Please consider this as the argument
> #3.
> 
> #1 is: conceptually, preferred cpumask improves performance because it
> moves the scheduling decision from host to the guest level. The guest
> can do it better because it knows more about the workload. But then we'd
> consider moving steal governor logic to the guest userspace, because it
> knows the nature of user workload even more.

I am confused; it is a simple control loop and has nothing to do with
guest workload.

The host is over-committed, this results in vcpu steal time. The guests
observe steal time and reduce vcpu utilization.

It is a co-operative scheme; a bad actor can game the system. It is
imperative all guests behave the same or do not play at all.

If there is no level of trust (our coconut friends) do not use this.

> #2: convenience and scope. Some systems don't allow loading modules -
> either on config level, through selinux, or somehow else. They don't
> even have compiler installed sometimes. 

You hurt if you shoot your foot off; not my problem. If you run guests
like that you don't use this.

> > we already have cpusets that is more than capable of doing this.
> 
> Then why do we need the steal governor at all? We can steer all
> processes away from a given vCPU without any driver, only by playing
> with each individual process' affinity. For vCPU throttling, task
> isolation, or whatever purpose.

Different level of operation; different semantics. Cpusets is for strong
bounds on your own workloads, this preferred mask is weak bounds on the
system due to outside pressure.

In a way it isn't much different from ACPI idle injection.