Re: [RFC PATCH 0/7] sched: cpufreq: Remove magic margins

From: Lukasz Luba <lukasz.luba@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Qais Yousef <qyousef@layalina.io>,
	Chris Redpath <Chris.Redpath@arm.com>
Subject: Re: [RFC PATCH 0/7] sched: cpufreq: Remove magic margins
Date: Tue, 12 Sep 2023 12:51:52 +0100	[thread overview]
Message-ID: <f1b1b663-3a12-9e5d-932b-b3ffb5f02e14@arm.com> (raw)
In-Reply-To: <20230907201609.GC14243@noisy.programming.kicks-ass.net>

Hi Peter,

On 9/7/23 21:16, Peter Zijlstra wrote:
> On Thu, Sep 07, 2023 at 03:42:13PM +0100, Lukasz Luba wrote:
> 
>>> What task characteristic is tied to this? That is, this seems trivial to
>>> modify per-task.
>>
>> In particular Speedometer test and the main browser task, which reaches
>> ~900util, but sometimes vanish and waits for other background tasks
>> to do something. In the meantime it can decay and wake-up on
>> Mid/Little (which can cause a penalty to score up to 5-10% vs. if
>> we pin the task to big CPUs). So, a longer util_est helps to avoid
>> at least very bad down migration to Littles...
> 
> Do they do a few short activations (wakeup/sleeps) while waiting? That
> would indeed completely ruin things since the EWMA thing is activation
> based.
> 
> I wonder if there's anything sane we can do here...

My apologies for a delay, I have tried to push the graphs for you.

The experiment is on pixel7*. It's running the browser on the phone
with the test 'Speedometer 2.0'. It's a web test (you can also run on
your phone) available here, no need to install anything:
https://browserbench.org/Speedometer2.0/

Here is the Jupiter notebook [1], with plots of the signals:
- top 20 tasks' (based on runtime) utilization
- Util EST signals for the top 20 tasks, with the longer decaying ewma
   filter (which is the 'red' plot called 'ewma')
- the main task (comm=CrRendererMain) Util, Util EST and task residency
   (which tires to stick to CPUs 6,7* )
- the test score was 144.6 (while with fast decay ewma is ~134), so
   staying at big cpus (helps the score in this case)

(the plots are interactive, you can zoom in with the icon 'Box Zoom')
(e.g. you can zoom in the task activation plot which is also linked
with the 'Util EST' on top, for this main task)

You can see the util signal of that 'CrRendererMain' task and those
utilization drops in time, which I was referring to. When the util
drops below some threshold, the task might 'fit' into smaller CPU,
which could be prevented automatically byt maintaining the util est
for longer (but not for all).

I do like your idea that Util EST might be per-task. I'm going to
check this, because that might help to get rid of the overutilized state
which is probably because small tasks are also 'bigger' for longer.

If this util est have chance to fly upstream, I could send an RFC if
you don't mind.

Regards,
Lukasz

*CPUs 6,7 - big (1024 capacity), CPUs 4,5 Mid (768 capacity), CPUs 0-3
Littles (~150 capacity)

[1] 
https://nbviewer.org/github/lukaszluba-arm/lisa/blob/public_tests/p7_wa_speedometer2_small_size.ipynb