Re: [PATCH] arm64: dts: qcom: sc7180: Add 'sustainable_power' for CPU thermal zones

[Matthias Kaehlcke]

Hi Doug,

On Tue, Sep 01, 2020 at 01:19:10PM -0700, Doug Anderson wrote:
> Hi,
> 
> On Tue, Sep 1, 2020 at 10:07 AM Matthias Kaehlcke <mka@xxxxxxxxxxxx> wrote:
> >
> > On Thu, Aug 13, 2020 at 11:30:33AM -0700, Matthias Kaehlcke wrote:
> > > The 'sustainable_power' attribute provides an estimate of the sustained
> > > power that can be dissipated at the desired control temperature. One
> > > could argue that this value is not necessarily the same for all devices
> > > with the same SoC, which may have different form factors or thermal
> > > designs. However there are reasons to specify a (default) value at SoC
> > > level for SC7180: most importantly, if no value is specified at all the
> > > power_allocator thermal governor (aka 'IPA') estimates a value, using the
> > > minimum power of all cooling devices of the zone, which can result in
> > > overly aggressive thermal throttling. For most devices an approximate
> > > conservative value should be more useful than the minimum guesstimate
> > > of power_allocator. Devices that need a different value can overwrite
> > > it in their <device>.dts. Also the thermal zones for SC7180 have a high
> > > level of granularity (essentially one for each function block), which
> > > makes it more likely that the default value just works for many devices.
> > >
> > > The values correspond to 1901 MHz for the big cores, and 1804 MHz for
> > > the small cores. The values were determined by limiting the CPU
> > > frequencies to different max values and launching a bunch of processes
> > > that cause high CPU load ('while true; do true; done &' is simple and
> > > does a good job). A frequency is deemed sustainable if the CPU
> > > temperatures don't rise (consistently) above the second trip point
> > > ('control temperature', 95 degC in this case). Once the highest
> > > sustainable frequency is found, the sustainable power can be calculated
> > > by multiplying the energy consumption per core at this frequency (which
> > > can be found in /sys/kernel/debug/energy_model/) with the number of
> > > cores that are specified as cooling devices.
> > >
> > > The sustainable frequencies were determined at room temperature
> > > on a device without heat sink or other passive cooling elements.
> 
> I'm curious: was this a bare board, or a device in a case?  Hrm, I'm
> not sure which one would be worse at heat dissipation, but I would
> imagine that being inside a plastic case might be worse?

This was with a device in a plastic case.

> > > Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
> > > ---
> > > If maintainers think 'sustainable_power' should be specified at
> > > device level (with which I conceptually agree) I'm fine with
> > > doing that, just seemed it could be useful to have a reasonable
> > > 'default' at SoC level in this case.
> >
> > Any comments on this?
> 
> I'm not massively familiar with this area of the code, but I guess I
> shouldn't let that stop me from having an opinion!  :-P
> 
> * I would agree that it seems highly unlikely that someone would put
> one of these chips in a device that could only dissipate the heat from
> the lowest OPP, so having some higher estimate definitely makes sense.
> 
> * In terms of the numbers here, I believe that you're claiming that we
> can dissipate 768 mW * 6 + 1202 mW * 2 = ~7 Watts of power.

No, I'm claiming it's 768 mW + 1202 mW = ~2 W.

SC7180 has a 6 thermal zones for the 6 little cores and 4 zones for the
2 big cores. Each of these thermal zones uses either all little or all big
cores as cooling devices, hence the power sustainable power of the
individual zones doesn't add up. 768 mW corresponds to 6x 128 mW (aka all
little cores at 1.8 GHz), and 1202 mW to 2x 601 mW (both big cores at 1.9 GHz).

> My memory
> of how much power we could dissipate in previous laptops I worked on
> is a little fuzzy, but that doesn't seem insane for a passively-cooled
> laptop.  However, I think someone could conceivably put this chip in a
> smaller form factor.  In such a case, it seems like we'd want these
> things to sum up to ~2000 (if it would ever make sense for someone to
> put this chip in a phone) or ~4000 (if it would ever make sense for
> someone to put this chip in a small tablet).

See above, the sustainable power with this patch only adds up to ~2000.
It is possible though that it would be lower in a smaller form factor
device.

I'd be ok with posting something lower for SC7180 (it would be a guess
though) and use the specific numbers in the device specific DT.

> It seems possible that,
> to achieve this, we might have to tweak the
> "dynamic-power-coefficient".  I don't know how much thought was put
> into those numbers, but the fact that the little cores have a super
> round 100 for their dynamic-power-coefficient makes me feel like they
> might have been more schwags than anything.  Rajendra maybe knows?

Yeah, it's possible that that was just an approximation

> * I'm curious about the fact that there are two numbers here: one for
> littles and one for bigs.  If I had to guess I'd say that since all
> the cores are in one package so the contributions kinda need to be
> thought of together, right?  If we're sitting there thermally
> throttled then we'd want to pick the best perf-per-watt for the
> overall package.  This is why your patch says we can sustain the
> little cores at max and the big cores get whatever is left over,
> right?

It's derived from how Qualcomm specified the thermal zones and cooling
devices. Any ("cpu") zone is either cooled by (all) big cores or by (all)
little cores, but not a mix of them. In my tests I also saw that the big
cores seemed to have little impact on the little ones. The little cores
are at max because even running at max frequency the temperature in the
'little zones' wouldn't come close to the trip point.

> * Should we be leaving some room in here for the GPU?  ...or I guess
> once we list it as a cooling device we'll have to decrease the amount
> the CPUs can use?

I don't know for sure, but judging from the CPU zones I wouldn't be
surprised if the GPU was managed exclusively in the dedicated GPU
thermal zones (I guess that's what 'gpuss0-thermal' and 'gpuss1-thermal'
are). If that's not the case the values in the CPU zones can be
adjusted when specific data is available.

> So I guess the tl; dr is:
> 
> a) We should check "dynamic-power-coefficient" and possibly adjust.

ok, lets see if Rajendra can check if there is room for tweaking.

> b) I don't think the "conservative" by-default numbers should add up
> to 7 Watts.  I could be convinced that this chip is not intended for
> phones and thus we could have it add up to 4 Watts, but 7 Watts seems
> too much.

I suppose this is mostly addressed by my explications above, unless we
think that 2 Watts in CPU power might still be too aggressive as a
default.

Thanks

m.