linux-kernel - Re: [PATCH 1/6] cpuidle: teo: Increase util-threshold

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Date: Fri, 7 Jun 2024 10:35:53 +0100
From: Christian Loehle <christian.loehle@....com>
To: Dietmar Eggemann <dietmar.eggemann@....com>, linux-pm@...r.kernel.org,
 linux-kernel@...r.kernel.org, rafael@...nel.org
Cc: vincent.guittot@...aro.org, qyousef@...alina.io, peterz@...radead.org,
 daniel.lezcano@...aro.org, anna-maria@...utronix.de,
 kajetan.puchalski@....com, lukasz.luba@....com
Subject: Re: [PATCH 1/6] cpuidle: teo: Increase util-threshold

On 6/7/24 09:01, Dietmar Eggemann wrote:
> On 06/06/2024 11:00, Christian Loehle wrote:
>> Increase the util-threshold by a lot as it was low enough for some
>> minor load to always be active, especially on smaller CPUs.
> 
> We see the blocked part of the CPU utilization as something telling the
> task scheduler that the corresponding tasks might be runnable soon again
> on this CPU.
> 
> This model seems to be used here as well. I guess folks are still
> debating whether the amount of blocked utilization is a good enough
> indicator for the length of idle time.

Right, the blocked utilization is treated as an indicator that we will
be brought out of sleep by a non-timer wakeup.

> 
>> For small cap CPUs (Pixel6) the util threshold is as low as 1.
>> For CPUs of capacity <64 it is 0. So ensure it is at a minimum, too.
> 
> So before this threshold was 16 on a 1024 CPU, now it's 256?
> 
> A <= 200 CPU has now a threshold of 50.
> 
> Where do those numbers come from? Just from running another workload on
> a specific device?
> 
> [...]

More or less yes.
Kajetan identified two broad use-cases for the utilization-based state
bypass: Early utilization ramp-up and high utilization scenarios.
The reports made it clear that the former can't be handled with a
threshold for just a single value as it will be too aggressive in
sustained (non-ramp-up) workloads.
To be fair, with patches 5 and 6 of this series, the ramp-up is
also handled quite early by the intercepts logic itself.
So as a fix I increased the value high enough to not trigger in
low-utilization scenarios.
There is likely room for optimization here, e.g. many wakeups
are also IPIs more related to the general system utilization instead
of the current CPU.