Re: [RFC PATCH 1/6] sched/fair: Add util_guest for tasks

[Quentin Perret]

On Monday 03 Apr 2023 at 18:11:37 (-0700), David Dai wrote:
> On Mon, Apr 3, 2023 at 4:40 AM Dietmar Eggemann
> <dietmar.eggemann@xxxxxxx> wrote:
> > I can't see why the existing p->uclamp_req[UCLAMP_MIN].value can't be
> > used here instead p->se.avg.util_guest.
> Using p->uclamp_req[UCLAMP_MIN].value would result in folding in
> uclamp values into task_util and task_util_est for all tasks that have
> uclamp values set. The intent of these patches isn’t to modify
> existing uclamp behaviour. Users would also override util values from
> the guest when they set uclamp values.

That shouldn't be a problem if host userspace is responsible for driving
the uclamp values in response to guest frequency requests in the first
place ...

> > I do understand the issue of inheriting uclamp values at fork but don't
> > get the not being `additive` thing. We are at task level here.
> Uclamp values are max aggregated with other tasks at the runqueue
> level when deciding CPU frequency. For example, a vCPU runqueue may
> have an util of 512 that results in setting 512 to uclamp_min on the
> vCPU task. This is insufficient to drive a frequency response if it
> shares the runqueue with another host task running with util of 512 as
> it would result in a clamped util value of 512 at the runqueue(Ex. If
> a guest thread had just migrated onto this vCPU).

Maybe it's a feature rather than bug?

It's not obvious giving extra powers to vCPU threads that other host
threads don't have is a good idea. The fact that vCPU threads are
limited to what the VMM would be allowed to request for its other
threads is more than desirable. I'd even say it's a requirement.

Thanks,
Quentin