Re: [PATCH v5 13/24] virt/steal_monitor: Add documentation

[Peter Zijlstra]

On Thu, Jun 25, 2026 at 06:16:37PM +0530, Shrikanth Hegde wrote:

> +Core idea:
> +==========
> +steal time is an indication available today in Guest which shows contention
> +for underlying physical CPU. Use it as a hint in the guest to fold the
> +workload to a reduced set of vCPUs. When there is contention, steal time
> +will show up in all the guests. When each guest honors the hint and folds
> +the workload to a smaller set of vCPUs(Preferred CPUs), it reduces the
> +contention and thereby reduces vCPU preemption.
> +This is achieved without any cross-guest communication.
> +
> +Steal monitor driver effectively does:
> +
> +1. Periodically computes steal time across the system.
> +
> +2. If steal time is greater than high threshold, reduce the number of
> +   preferred CPUs by 1 core. Ensure at least one core is left always.
> +   This avoids running into extreme cases.
> +
> +3. If steal time is lower or equal to low threshold, increase the
> +   number of preferred CPUs by 1 core. If preferred is same as active,
> +   nothing to be done.
> +
> +4. Ensure preferred CPUs is always subset of active CPUs.
> +   On feature disable it is same as active CPUs.


So this is very much a co-operative scheme. Perhaps add a few words to
describe the effect of a non cooperative guest. IIRC the result is not
worse than the status quo. That is, if one (or more) guests refuse to
co-operate it will not make things worse, it will just not result in
improvements, right?