Re: [PATCH v2 1/1] x86,sched: On AMD EPYC set freq_max = max_boost in schedutil invariant formula

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jan 22, 2021 at 09:40:38PM +0100, Giovanni Gherdovich wrote:
> 1. PROBLEM DESCRIPTION (over-utilization and schedutil)
> 
> The problem happens on CPU-bound workloads spanning a large number of cores.
> In this case schedutil won't select the maximum P-State. Actually, it's
> likely that it will select the minimum one.
> 
> A CPU-bound workload puts the machine in a state generally called
> "over-utilization": an increase in CPU speed doesn't result in an increase of
> capacity. The fraction of time tasks spend on CPU becomes constant regardless
> of clock frequency (the tasks eat whatever we throw at them), and the PELT
> invariant util goes up and down with the frequency (i.e. it's not invariant
> anymore).

>                                       v5.10          v5.11-rc4
>                                       ~~~~~~~~~~~~~~~~~~~~~~~~
> CPU activity (mpstat)                 80-90%         80-90%
> schedutil requests (tracepoint)       always P0      mostly P2
> CPU frequency (HW feedback)           ~2.2 GHz       ~1.5 GHz
> PELT root rq util (tracepoint)        ~825           ~450
> 
> mpstat shows that the workload is CPU-bound and usage doesn't change with

So I'm having trouble with calling a 80%-90% workload CPU bound, because
clearly there's a ton of idle time.