linux-kernel - Re: [PATCH v4 10/18] PM: EM: Add RCU mechanism which safely cleans the old data

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Date:   Wed, 11 Oct 2023 18:07:21 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Wei Wang <wvw@...gle.com>
Cc:     Lukasz Luba <lukasz.luba@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        dietmar.eggemann@....com, rui.zhang@...el.com,
        amit.kucheria@...durent.com, amit.kachhap@...il.com,
        daniel.lezcano@...aro.org, viresh.kumar@...aro.org,
        len.brown@...el.com, pavel@....cz, mhiramat@...nel.org,
        qyousef@...alina.io
Subject: Re: [PATCH v4 10/18] PM: EM: Add RCU mechanism which safely cleans
 the old data

On Wed, Oct 11, 2023 at 6:03 PM Wei Wang <wvw@...gle.com> wrote:
>
> On Fri, Oct 6, 2023 at 1:45 AM Lukasz Luba <lukasz.luba@....com> wrote:
> >
> > Hi Rafael,
> >
> > A change of direction here, regarding your comment below.
> >
> > On 10/2/23 14:44, Lukasz Luba wrote:
> > >
> > >
> > > On 9/29/23 13:59, Rafael J. Wysocki wrote:
> > >> On Fri, Sep 29, 2023 at 11:36 AM Lukasz Luba <lukasz.luba@....com> wrote:
> > >
> > > [snip]
> > >
> >
> > [snip]
> >
> > >>>> Apparently, some frameworks are only going to use the default table
> > >>>> while the runtime-updatable table will be used somewhere else at the
> > >>>> same time.
> > >>>>
> > >>>> I'm not really sure if this is a good idea.
> > >>>
> > >>> Runtime table is only for driving the task placement in the EAS.
> > >>>
> > >>> The thermal gov IPA won't make better decisions because it already
> > >>> has the mechanism to accumulate the error that it made.
> > >>>
> > >>> The same applies to DTPM, which works in a more 'configurable' way,
> > >>> rather that hard optimization mechanism (like EAS).
> > >>
> > >> My understanding of the above is that the other EM users don't really
> > >> care that much so they can get away with using the default table all
> > >> the time, but EAS needs more accuracy, so the table used by it needs
> > >> to be adjusted in certain situations.
> > >
> > > Yes
> > >
> > >>
> > >> Fair enough, I'm assuming that you've done some research around it.
> > >> Still, this is rather confusing.
> > >
> > > Yes, I have presented those ~2y ago in Android Gerrit world
> > > (got feedback from a few vendors) and in a few Linux conferences.
> > >
> > > For now we don't plan to have this feature for the thermal
> > > governor or something similar.
> > >
> >
> > I have discussed with one of our partners your comment about 2 tables.
> > They would like to have this runtime modified EM in other places
> > as well: DTPM and thermal governor. So you had good gut feeling.
> >
> > In the past in our IPA (thermal gov ~2016 and kernel v4.14) we
> > had two callbacks:
> > - get_static_power() [1]
> > - get_dynamic_power() [2]
> >
> > Later ~2017/2018 v4.16 the static power mechanism was removed
> > completely by this commit 84fe2cab48590e4373978e4e.
> > The way how it was design, implemented and used justified that
> > decision. We later used EM in the cpu cooling which also only
> > had dynamic power information.
> >
> > The PID mechanism in IPA tries to compensate that
> > missing information (about changed static power in time or a chip
> > binning) and adjusts the 'error'. How good and fast that is in all
> > situations - it's a different story (out of this scope).
> > So, IPA should not be worse with the runtime table.
> >
> > The static power was on the chips and probably will be still.
> > You might remember my slide 13 from OSPM2024 showing two power
> > usage plots for the same Big CPU and 1.4GHz fixed (50% of fmax):
> > - w/ GPU working in the background using 1-1.5W
> > - w/o GPU in the background
> >
> > The same workload run on Big, but power bigger is ~15% higher
> > after ~1min.
> >
> > The static power (leakage) is the issue that this patch tries
> > to address for EAS. Although, there is not only the leakage.
> > It's about the whole 'profile', which can be different than what
> > could be built during boot default information.
> >
> > So we would want to go for one single table in EM, which
> > is runtime modifiable.
> >
> > That is something that you might be more confident and we would
> > have less diversity (2 tables) in the kernel.
> >
> > Regards,
> > Lukasz
> >
> >
>
> Indeed, we had a conversation about this with Lukasz recently. The key
> idea is that there is no compelling reason to introduce diversity in
> the mathematics involved. If we have confidence in the superior
> accuracy of our model, it should be universally implemented. While the
> governors are designed with some error tolerance, they can benefit
> from enhanced accuracy in their operation.

I agree, thanks!

> > [1]
> > https://elixir.bootlin.com/linux/v4.14/source/drivers/thermal/cpu_cooling.c#L336
> > [2]
> > https://elixir.bootlin.com/linux/v4.14/source/drivers/thermal/cpu_cooling.c#L383