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Date: Mon, 15 Jan 2024 12:21:56 +0000
From: Qais Yousef <qyousef@layalina.io>
To: Lukasz Luba <lukasz.luba@arm.com>
Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
	rafael@kernel.org, dietmar.eggemann@arm.com, rui.zhang@intel.com,
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	daniel.lezcano@linaro.org, viresh.kumar@linaro.org,
	len.brown@intel.com, pavel@ucw.cz, mhiramat@kernel.org,
	wvw@google.com
Subject: Re: [PATCH v5 15/23] PM: EM: Optimize em_cpu_energy() and remove
 division
Message-ID: <20240115122156.5743y4trhm4tkgs3@airbuntu>
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On 01/10/24 13:53, Lukasz Luba wrote:
> 
> 
> On 1/4/24 19:23, Qais Yousef wrote:
> > On 01/02/24 11:47, Lukasz Luba wrote:
> > > > Did you see a problem or just being extra cautious here?
> > > 
> > > There is no problem, 'cost' is a private coefficient for EAS only.
> > 
> > Let me  ask differently, what goes wrong if you don't increase the resolution
> > here? Why is it necessary?
> > 
> 
> 
> When you have 800mW at CPU capacity 1024, then the value is small (below
> 1 thousand).
> Example:
> power = 800000 uW
> cost = 800000 / 1024 = 781
> 
> While I know from past that sometimes OPPs might have close voltage
> values and a rounding could occur and make some OPPs inefficient
> while they aren't.
> 
> This is what would happen when we have the 1x resolution:
> /sys/kernel/debug/energy_model/cpu4/ps:1008000/cost:551
> /sys/kernel/debug/energy_model/cpu4/ps:1200000/cost:644
> /sys/kernel/debug/energy_model/cpu4/ps:1416000/cost:744
> /sys/kernel/debug/energy_model/cpu4/ps:1512000/cost:851
> /sys/kernel/debug/energy_model/cpu4/ps:408000/cost:493
> /sys/kernel/debug/energy_model/cpu4/ps:600000/cost:493
> /sys/kernel/debug/energy_model/cpu4/ps:816000/cost:493
> The bottom 3 OPPs have the same 'cost' thus 2 OPPs are in-efficient,
> which is not true (see below).
> 
> This is what would happen when we have the 10x resolution:
> /sys/kernel/debug/energy_model/cpu4/ps:1008000/cost:5513
> /sys/kernel/debug/energy_model/cpu4/ps:1200000/cost:6443
> /sys/kernel/debug/energy_model/cpu4/ps:1416000/cost:7447
> /sys/kernel/debug/energy_model/cpu4/ps:1512000/cost:8514
> /sys/kernel/debug/energy_model/cpu4/ps:408000/cost:4934
> /sys/kernel/debug/energy_model/cpu4/ps:600000/cost:4933
> /sys/kernel/debug/energy_model/cpu4/ps:816000/cost:4934
> Here the OPP with 600MHz is more efficient than 408MHz,
> which is true. So only 408MHz will be marked as in-efficient OPP.
> 
> 
> This is what would happen when we have the 100x resolution:
> /sys/kernel/debug/energy_model/cpu4/ps:1008000/cost:55137
> /sys/kernel/debug/energy_model/cpu4/ps:1200000/cost:64433
> /sys/kernel/debug/energy_model/cpu4/ps:1416000/cost:74473
> /sys/kernel/debug/energy_model/cpu4/ps:1512000/cost:85140
> /sys/kernel/debug/energy_model/cpu4/ps:408000/cost:49346
> /sys/kernel/debug/energy_model/cpu4/ps:600000/cost:49331
> /sys/kernel/debug/energy_model/cpu4/ps:816000/cost:49346
> The higher (100x) resolution does not bring that much in
> practice.

So it seems a uW is not sufficient enough. We moved from mW because of
resolution already. Shall we make it nW then and multiply by 1000 always? The
choice of 10 looks arbitrary IMHO

> 
> If you have other questions, let's continue on v6 series.