linux-kernel - Re: [RFC PATCH 08/16] sched/fair: Extend util

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Message-ID: <d67d420f-8cd4-4b56-8a93-ebbd513268e8@arm.com>
Date: Mon, 14 Oct 2024 17:04:34 +0100
From: Christian Loehle <christian.loehle@....com>
To: Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Viresh Kumar <viresh.kumar@...aro.org>
Cc: Juri Lelli <juri.lelli@...hat.com>, Steven Rostedt <rostedt@...dmis.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>, John Stultz
 <jstultz@...gle.com>, linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 08/16] sched/fair: Extend util_est to improve rampup
 time

On 8/20/24 17:35, Qais Yousef wrote:
> Utilization invariance can cause big delays. When tasks are running,
> accumulate non-invairiant version of utilization to help tasks to settle
> down to their new util_avg values faster.
> 
> Keep track of delta_exec during runnable across activations to help
> update util_est for a long running task accurately. util_est shoudl
> still behave the same at enqueue/dequeue.

For periodic tasks that have longer slices (~tick) this overestimates
util_est by a lot.
AFAICS this also breaks util_est for co-scheduling tasks of different slice
lengths.

I'm testing with HZ=1000, but should work for any. On a RK3399, all pinned
to one big.

Having task A be 10ms period with 20% util (running for 2ms when scheduled)
and tasks B+ with 1ms period and 1% util.

I guess 9/16 tries to work around that somewhat but without any leeway that
doesn't work. Even rt-app tasks will vary slightly in their util_est values:

Task A only:
mainline:
A:
util_avg: 192 util_est: 204

9/16 sched/fair: util_est: Take into account periodic tasks
A:
util_avg: 185 util_est: 423

8 tasks:
mainline:
A:
util_avg: 229 util_est: 229
The rest
util_avg:  12 util_est:  24

9/16 sched/fair: util_est: Take into account periodic tasks
A:
util_avg: 242 util_est: 643
The rest:
util_avg:  12 util_est:  50