Re: [RFC/RFT][PATCH] cpufreq: schedutil: Reduce frequencies slower

[Rafael J. Wysocki]

On Friday, March 31, 2017 11:22:23 AM Juri Lelli wrote:
> Hi Rafael,

Hi Juri,

> On 30/03/17 23:36, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > 
> > The schedutil governor reduces frequencies too fast in some
> > situations which cases undesirable performance drops to
> > appear.
> > 
> > To address that issue, make schedutil reduce the frequency slower by
> > setting it to the average of the value chosen during the previous
> > iteration of governor computations and the new one coming from its
> > frequency selection formula.
> > 
> 
> I'm curious to test this out on Pixel phones once back in office, but
> I've already got some considerations about this patch. Please find them
> inline below.
> 
> > Link: https://bugzilla.kernel.org/show_bug.cgi?id=194963
> > Reported-by: John <john.ettedgui@xxxxxxxxx>
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > ---
> > 
> > This addresses a practical issue, but one in the "responsiveness" or
> > "interactivity" category which is quite hard to represent quantitatively.
> > 
> > As reported by John in BZ194963, schedutil does not ramp up P-states quickly
> > enough which causes audio issues to appear in his gaming setup.  At least it
> > evidently is worse than ondemand in this respect and the patch below helps.
> > 
> 
> Might also be a PELT problem?

I don't think so.

As mentioned below, intel_pstate had it too and it doesn't use PELT. :-)

This appears to be a general issue with load-based (or utilization-based)
frequency selection algorithms using periodic sampling.  Roughly, if something
unanticipated is going to happen shortly (such as a burst in audio activity in
a game), it may take a whole period to notice what's going on and the frequency
set for that period can make a difference between sufficient and insufficient
provisioning.

What the patch does is to increase the likelihood that the frequency in
question will be sufficient to avoid noticeable effects (such as audio cracks)
and it tends to do the trick most of the time.

[Of course, you may argue that this is related to the rate limitting in
 schedutil and intel_pstate, but then PELT itself is sampled periodically
 AFAICS.]

> > The patch essentially repeats the trick added some time ago to the load-based
> > P-state selection algorithm in intel_pstate, which allowed us to make it viable
> > for performance-oriented users, and which is to reduce frequencies at a slower
> > pace.
> > 
> > The reason why I chose the average is because it is computationally cheap
> > and pretty much the max reasonable slowdown and the idea is that in case
> > there's something about to run that we don't know about yet, it is better to
> > stay at a higher level for a while more to avoid having to get up from the floor
> > every time.
> > 
> 
> Another approach we have been playing with on Android (to solve what
> seem to me similar issues) is to have decoupled up and down frequency
> changes thresholds. With this you can decide how much quick to react to
> a sudden increase in utilization and how much "hysteresis" you want to
> have before slowing down. Every platfrom can also be easily tuned as
> needed (instead of having the same filter applied for every platform).
> 
> We seemed to actually recently come to the consideration that the up
> threshold is probably not much needed (and it is in fact set to very
> small values in practice). Once one is confident that the utilization
> signal is not too jumpy, responding to a request for additional capacity
> quickly seems the right thing to do (especially for RT/DL policies).
> 
> What's your opinion?

As I said, responding to increased load may take a whole period to notice
and it looks like what happens during that period may be quite important.

To me, thresholds have a problem that from the algorithm perspective they
are constant values set externally.  This means they likely need to be tuned
once in a while by whatever entity that had set them (it is difficult to
imagine that the same values will always be suitable for every workload) and
that means an additional layer of (dynamic) control on top of the governor.

> > But technically speaking it is a filter. :-)
> > 
> > So among other things I'm wondering if that leads to substantial increases in
> > energy consumption anywhere.
> > 
> 
> Having a tunable might help getting the tradeoff right for different
> platforms, maybe?

It might, but it would mean additional computational cost (at least one more
integer multiplication AFAICS).

> As we discussed at the last LPC, having an energy model handy and use
> that to decide how quickly to ramp up or slow down seems the desirable
> long term solution, but we probably need something (as you are
> proposing) until we get there.

Well, we definitely need something to address real use cases, like the one that
I responded to with this patch. :-)

Thanks,
Rafael