Re: [LSF/MM/BPF TOPIC] Discuss more features + use cases for sched_ext

[Tejun Heo <tj@kernel.org>]

Hello, Joel.

On Mon, Jan 29, 2024 at 05:42:54PM -0500, Joel Fernandes wrote:
> > This is a great topic. I think integrating/merging such mechanism with the NEST
> > scheduler could be useful too? You mentioned there is sched_ext implementation
> > of NEST already? One reason that's interesting to me is the task-packing and
> > less-spreading may have power benefits, this is exactly what EAS on ARM does,
> > but it also uses an energy model to know when packing is a bad idea. Since we
> > don't have fine grained control of frequency on Intel, I wonder what else can we
> > do to know when the scheduler should pack and when to spread. Maybe something
> > simple which does not need an energy model but packs based on some other
> > signal/heuristic would be great in the short term.
> > 
> > Maybe a signal can be the "Quality of service (QoS)" approach where tasks with
> > lower QoS are packed more aggressively and higher QoS are spread more (?).

This was done for a different purpose (improving tail latencies on latency
critical workload) but it uses soft-affinity based packing which maybe can
translate to power-aware scheduling:

  https://github.com/sched-ext/scx/blob/case-studies/case-studies/scx_layered.md

I have a raptor lake-H laptop which has E and P cores and by default the
threads are being spread across all CPUs which probably isn't best for power
consumption. I was thinking about writing a scheduler which uses a similar
strategy as scx_layered - pack the cores one by one overflowing to the next
core from E to P when the average utilization crosses a set threshold. Most
of the logic is already in scx_layered, so maybe it can just be a part of
that. I'm curious whether whether and how much power can be saved with a
generic approach like that.

Thanks.

-- 
tejun