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

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, Jan 26, 2021 at 10:09:27AM +0100, Giovanni Gherdovich wrote:
> On Mon, 2021-01-25 at 11:06 +0100, Peter Zijlstra wrote:
> > 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.
> 
> Yes you're right. There is considerable idle time and calling it CPU-bound is
> a bit of a stretch.
> 
> Yet I don't think I'm completely off the mark. The busy time is the same with
> the machine running at 1.5 GHz and at 2.2 GHz (it just takes longer to
> finish). To me it seems like the CPU is the bottleneck, with some overhead on
> top.
> 

I think this is an important observation because while the load may not
be fully CPU-bound, it's still at the point where race-to-idle by running
at a higher frequency is relevant. During the busy time, the results
(and Michael's testing) indicate that the higher frequency may still be
justified. I agree that there is a "a 'problem' between schedutil and
cpufreq, they don't use the same f_max at all times", fixing that mid
-rc may not be appropriate because it's a big change in an rc window.

So, should this patch be merged for 5.11 as a stopgap, fix up
schedutil/cpufreq and then test both AMD and Intel chips reporting the
correct max non-turbo and max-turbo frequencies? That would give time to
give some testing in linux-next before merging to reduce the chance
something else falls out.

-- 
Mel Gorman
SUSE Labs