linux-kernel - Re: [PATCH 4/4] arm64: dts: rockchip: Add OPP data for CPU cores on RK3588

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Message-ID: <CABjd4YzdD9ciMn=p=opEK+fdxCkeCodsryph7pkqgsEUNcNrUQ@mail.gmail.com>
Date: Fri, 26 Jan 2024 17:44:46 +0400
From: Alexey Charkov <alchark@...il.com>
To: Daniel Lezcano <daniel.lezcano@...aro.org>
Cc: Dragan Simic <dsimic@...jaro.org>, Rob Herring <robh+dt@...nel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>, 
	Heiko Stuebner <heiko@...ech.de>, devicetree@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-rockchip@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 4/4] arm64: dts: rockchip: Add OPP data for CPU cores on RK3588

On Fri, Jan 26, 2024 at 4:56 PM Daniel Lezcano
<daniel.lezcano@...aro.org> wrote:
>
> On 26/01/2024 08:49, Dragan Simic wrote:
> > On 2024-01-26 08:30, Alexey Charkov wrote:
> >> On Fri, Jan 26, 2024 at 11:05 AM Dragan Simic <dsimic@...jaro.org> wrote:
> >>> On 2024-01-26 07:44, Alexey Charkov wrote:
> >>> > On Fri, Jan 26, 2024 at 10:32 AM Dragan Simic <dsimic@...jaro.org>
> >>> > wrote:
> >>> >> On 2024-01-25 10:30, Daniel Lezcano wrote:
> >>> >> > On 24/01/2024 21:30, Alexey Charkov wrote:
> >>> >> >> By default the CPUs on RK3588 start up in a conservative
> >>> performance
> >>> >> >> mode. Add frequency and voltage mappings to the device tree to
> >>> enable
>
> [ ... ]
>
> >> Throttling would also lower the voltage at some point, which cools it
> >> down much faster!
> >
> > Of course, but the key is not to cool (and slow down) the CPU cores too
> > much, but just enough to stay within the available thermal envelope,
> > which is where the same-voltage, lower-frequency OPPs should shine.
>
> That implies the resulting power is sustainable which I doubt it is the
> case.
>
> The voltage scaling makes the cooling effect efficient not the frequency.
>
> For example:
>         opp5 = opp(2GHz, 1V) => 2 BogoWatt
>         opp4 = opp(1.9GHz, 1V) => 1.9 BogoWatt
>         opp3 = opp(1.8GHz, 0.9V) => 1.458 BogoWatt
>         [ other states but we focus on these 3 ]
>
> opp5->opp4 => -5% compute capacity, -5% power, ratio=1
> opp4->opp3 => -5% compute capacity, -23.1% power, ratio=21,6
>
> opp5->opp3 => -10% compute capacity, -27.1% power, ratio=36.9
>
> In burst operation (no thermal throttling), opp4 is pointless we agree
> on that.
>
> IMO the following will happen: in burst operation with thermal
> throttling we hit the trip point and then the step wise governor reduces
> opp5 -> opp4. We have slight power reduction but the temperature does
> not decrease, so at the next iteration, it is throttle at opp3. And at
> the end we have opp4 <-> opp3 back and forth instead of opp5 <-> opp3.
>
> It is probable we end up with an equivalent frequency average (or
> compute capacity avg).
>
> opp4 <-> opp3 (longer duration in states, less transitions)
> opp5 <-> opp3 (shorter duration in states, more transitions)
>
> Some platforms had their higher OPPs with the same voltage and they
> failed to cool down the CPU in the long run.
>
> Anyway, there is only one way to check it out :)
>
> Alexey, is it possible to compare the compute duration for 'dhrystone'
> with these voltage OPP and without ? (with a period of cool down between
> the test in order to start at the same thermal condition) ?

Sure, let me try that - would be interesting to see the results. In my
previous tinkering there were cases when the system stayed at 2.35GHz
for all big cores for non-trivial time (using the step-wise thermal
governor), and that's an example of "same voltage, lower frequency".
Other times though it throttled one cluster down to 1.8GHz and kept
the other at 2.4GHz, and was also stationary at those parameters for
extended time. This probably indicates that both of those states use
sustainable power in my cooling setup.

Note though that I still have that tiny heatsink installed (even
though I disable the fan during tests), and in this setup the
temperature drops from 85C to around 70C in a matter of seconds as
soon as the load stops. And if I enable the fan then it balances the
temperature at the control setpoint of 55C using less than full fan
speed with 8 threads of dhrystone running for extended time (and the
PWM value chosen by the step-wise governor stabilizes at 240 out of
255). Looks like my prior assessment that "the fan is not super mighty
vs. the total thermal output" was wrong after all, despite its modest
size :)

Best regards,
Alexey