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

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Date: Sun, 16 Jun 2024 22:54:26 +0100
From: Qais Yousef <qyousef@...alina.io>
To: Christian Loehle <christian.loehle@....com>
Cc: linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	rafael@...nel.org, vincent.guittot@...aro.org, peterz@...radead.org,
	daniel.lezcano@...aro.org, anna-maria@...utronix.de,
	kajetan.puchalski@....com, lukasz.luba@....com,
	dietmar.eggemann@....com
Subject: Re: [PATCH 1/6] cpuidle: teo: Increase util-threshold

On 06/10/24 10:11, Christian Loehle wrote:
> On 6/9/24 23:47, Qais Yousef wrote:
> > On 06/06/24 10:00, 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.
> >>
> >> 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.
> >>
> >> Fixes: 9ce0f7c4bc64 ("cpuidle: teo: Introduce util-awareness")
> >> Reported-by: Qais Yousef <qyousef@...alina.io>
> >> Reported-by: Vincent Guittot <vincent.guittot@...aro.org>
> >> Suggested-by: Kajetan Puchalski <kajetan.puchalski@....com>
> >> Signed-off-by: Christian Loehle <christian.loehle@....com>
> >> ---
> >>  drivers/cpuidle/governors/teo.c | 11 +++++------
> >>  1 file changed, 5 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/drivers/cpuidle/governors/teo.c b/drivers/cpuidle/governors/teo.c
> >> index 7244f71c59c5..45f43e2ee02d 100644
> >> --- a/drivers/cpuidle/governors/teo.c
> >> +++ b/drivers/cpuidle/governors/teo.c
> >> @@ -146,13 +146,11 @@
> >>   * The number of bits to shift the CPU's capacity by in order to determine
> >>   * the utilized threshold.
> >>   *
> >> - * 6 was chosen based on testing as the number that achieved the best balance
> >> - * of power and performance on average.
> >> - *
> >>   * The resulting threshold is high enough to not be triggered by background
> >> - * noise and low enough to react quickly when activity starts to ramp up.
> >> + * noise.
> >>   */
> >> -#define UTIL_THRESHOLD_SHIFT 6
> >> +#define UTIL_THRESHOLD_SHIFT 2
> >> +#define UTIL_THRESHOLD_MIN 50
> >>  
> >>  /*
> >>   * The PULSE value is added to metrics when they grow and the DECAY_SHIFT value
> >> @@ -671,7 +669,8 @@ static int teo_enable_device(struct cpuidle_driver *drv,
> >>  	int i;
> >>  
> >>  	memset(cpu_data, 0, sizeof(*cpu_data));
> >> -	cpu_data->util_threshold = max_capacity >> UTIL_THRESHOLD_SHIFT;
> >> +	cpu_data->util_threshold = max(UTIL_THRESHOLD_MIN,
> >> +				max_capacity >> UTIL_THRESHOLD_SHIFT);
> > 
> > Thanks for trying to fix this. But I am afraid this is not a solution. There's
> > no magic number that can truly work here - we tried. As I tried to explain
> > before, a higher util value doesn't mean long idle time is unlikely. And
> > blocked load can cause problems where a decay can take too long.
> > 
> > We are following up with the suggestions I have thrown back then and we'll
> > share results if anything actually works.
> 
> Namely watching increase / decrease of utilization?

We still have to explore and see. I think we need multiple cues if we are to
try to predict the likelihood of a task waking up sooner than min_residency.
And be scalable across workloads/systems.

> I think you would have to watch at least a couple of values before entering such
> a logic and at that point the intercepts logic will handle it anyway.
> Furthermore IMO we should be wary about introducing any state in teo that persists
> across calls if not absolutely necessary (like intercept-detection) as it really
> makes teo much less predictable.
> 
> > 
> > For now, I think a revert is more appropriate. There was some perf benefit, but
> > the power regressions were bad and there's no threshold value that actually
> > works. The thresholding concept itself is incorrect and flawed - it seemed the
> > correct thing back then, yes. But in a hindsight now it doesn't work.
> 
> Unfortunate :/
> OK. I'll do some more testing with that, too. From what I can see a revert wouldn't
> have terrible fallout with the series altogether, so I might just change this for
> v2 and drop 2/6.
> 
> Kind Regards,
> Christian
>