Re: [PATCH] sched: Take thermal pressure into account when determine rt fits capacity

[Xuewen Yan <xuewen.yan94@gmail.com>]

On Wed, May 11, 2022 at 6:03 AM Lukasz Luba <lukasz.luba@arm.com> wrote:
>
>
>
> On 5/10/22 19:44, Qais Yousef wrote:
> > On 05/10/22 18:44, Lukasz Luba wrote:
> >
> > [...]
> >
> >> To properly answer this question we probably have to analyze the timings
> >> and this update path - how often it is actually called. Keep in mind
> >> we are going to solve CPU capacity inversion for RT class, which
> >> contains latency sensitive tasks. In this approach the information
> >
> > This was an attempt for a generic inversion detection. We update
> > rq->cpu_capacity which is used by capacity_of() in the same path.
>
> True, but this is a CFS 'world' and the update path is part of load
> balance. Your proposed code which sets the new
> 'rq->cpu_capacity_inverted' is run there, which might have some
> delays.

Yes, that's exactly what I'm worried about.

>
> >
> > I didn't feel brave to write a quick patch in the topology code, but we can
> > certainly do the detection there in topology_update_thermal_pressure().
>
> Looks better, since that code path is called when we get instantaneous
> information about CPU freq reduction. I'm afraid that again this
> approach might be blocked due to 'khz' calling ratio of that code and we
> 'must not' use this.
>
> >
> >> about HW status is coming from this CFS load balance path.
> >> What if that load balance is not called that often as RT might require?
> >> What if there is a light load on CPUs, but GPU caused them to throttle,
> >> reducing capacity by a decent chunk e.g. 50%?
> >> That would translate to some RT periodic task which takes 2ms every
> >> 8ms to take 4ms, while maybe on other less power hungry CPU it could
> >> take 3ms.
> >>
> >> The usage of thermal_load_avg() in the scale_rt_capacity() looks OK
> >> for the CFS, but might not be from the RT class point of view.
> >> The RT class might want to realize faster that CPUs have changed the
> >> capacity.
> >> Maybe it's OK with that patch [1] and boot config shifter=-5, but in
> >> default boot config for shifter=0 we can suffer for hundreds of ms
> >> running on lower capacity cpu (which is quite high number of frames
> >> nowadays).
> >>
> >> Without a research and experiments data I'm afraid this is too
> >> big step to make, with this CFS load balance path.
> >
> > I think Xuewen didn't want to use thermal_load_avg(), and that's the question
> > I deferred.
>
> Your code snipped might have similar penalty, since you populate
> information about that CPU inversion at 'some point in time'.
> My point is: that 'point in time' is not well defined, since it's
> CFS load balance. I'm afraid that RT class deserves something better
> defined (predictable, repeatable, reliable, short, etc.)
>
> >
> >>
> >>>
> >>>>
> >>>>> +
> >>>>> +               rq->cpu_capacity_inverted = 0;
> >>>>> +
> >>>>> +               for_each_possible_cpu(cpu) {
> >>>>> +                       unsigned long cap = arch_scale_cpu_capacity(cpu);
> >>>>> +
> >>>>> +                       if (capacity_orig <= cap)
> >>>>> +                               continue;
> >>
> >> The search loop here assumes that other CPUs (fortunately not in the
> >> same freq domain) don't suffer due to reduced capacity. This might be
> >> not true - when we have ~1 Watt budget for all CPUs in the system and
> >> single big core can use 3-4W at max or single mid core ~1.2W.
>
> s/1.2W/1-2W
>
> >
> > I defined capacity inversion against capacity_orig. IMHO that's the sensible
> > definition to make.
> >
> > Would be good to hear more/other suggestions.
>
> Capacity of other CPU might also be reduced and capacity_orig is not
> reflecting that. My gut feeling tells me that this capacity_orig
> assumption might be too optimistic for some platforms.

In unisoc platform with 3 clusters(little/mid/big), there are cases
that middle core and big core are throttled at the same time.

>
> It's the same old question: how good the model should be.
> We want to 'model' the reality (CPUs slows down), how good the
> model should be in this RT world use case - I don't know w/o
> experiments.
>
> I don't even know how often this new variable
> 'rq->cpu_capacity_inverted' gets updated and what is the time diff to
> the last update of the raw thermal pressure variable. You said that code
> is 'completely untested'. So it's unknown delay for now - but belongs to
> similar class as thermal_load_avg(), but the 2nd is known. I have
> shared plots with raw signal vs. PELT-like delays. We at least know
> the delays, e.g. ~200ms to reach raw value, but how that impacts RT
> world - I have no experiment results from real apps (i.e. w/ audio or
> display threads).
>
> >
> >>
> >>>>> +
> >>>>> +                       if (cap > inv_cap) {
> >>>>> +                               rq->cpu_capacity_inverted = inv_cap;
> >>>>> +                               break;
> >>>>> +                       }
> >>>>> +               }
> >>>>> +
> >>>>> +       }
> >>>>>
> >>>>>           sdg->sgc->capacity = capacity;
> >>>>>           sdg->sgc->min_capacity = capacity;
> >>>>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> >>>>> index 8dccb34eb190..bfe84c870bf9 100644
> >>>>> --- a/kernel/sched/sched.h
> >>>>> +++ b/kernel/sched/sched.h
> >>>>> @@ -992,6 +992,7 @@ struct rq {
> >>>>>
> >>>>>           unsigned long           cpu_capacity;
> >>>>>           unsigned long           cpu_capacity_orig;
> >>>>> +       unsigned long           cpu_capacity_inverted;
> >>>>>
> >>>>>           struct callback_head    *balance_callback;
> >>>>>
> >>>>> @@ -2807,6 +2808,11 @@ static inline unsigned long capacity_orig_of(int cpu)
> >>>>>           return cpu_rq(cpu)->cpu_capacity_orig;
> >>>>>    }
> >>>>>
> >>>>> +static inline unsigned long cpu_in_capacity_inversion(int cpu)
> >>>>> +{
> >>>>> +       return cpu_rq(cpu)->cpu_capacity_inverted;
> >>>>> +}
> >>>>> +
> >>>>>    /**
> >>>>>     * enum cpu_util_type - CPU utilization type
> >>>>>     * @FREQUENCY_UTIL:    Utilization used to select frequency
> >>>>>
> >>>>>
> >>>>> --->8---
> >>>>
> >>>> The patch is amazing for me, and the complexity is not too high. Would
> >>>> you please push the patch?
> >>>> I think the idea is yours, I don't want to use it as my patch v2.
> >>>
> >>> I'd be happy to add a commit message so that you can include it in your v2.
> >>>
> >>> First, I'd like to hear from Vincent and Lukasz they're happy with this
> >>> approach.
> >>>
> >>> I've been trying to think how we can do this generically but can't find an
> >>> alternative to the extra loop or additional fallback_cpu_mask. Maybe the mask
> >>> is okay if we protect it with sched_asymmetric_cpucapacity static key..
> >>>
> >>
> >> I'm sorry Qais, I see that you are trying to bring this
> >> real-CPU-capacity information into RT, but the source and quality of
> >> this information IMO might matter. I cannot help you w/o experiment
> >> results of your proposed approach.
> >
> > The question I was posing here is whether to handle thermal only in inversion
> > case as I was suggesting or do better. We are still trickling through the
> > details, but first, I wanted to make sure there's no objection to this
> > direction (detect inversion and bail out in rt_task_fits_capacity() for cpus in
> > capacity inversion).
>
> IMO how you detect that inversion and at which point in time is part of
> the scope.
>
> I would vote for using that thermal update code path + compare other
> CPUs real capacity not capacity_orig to detect inversion.

Okay, I could push patch v2 later. Maybe we can continue to discuss
this topic based on v2.

Thanks!
---
xuewen.yan