linux-kernel - [PATCH 0/4] Fix dvfs_headroom escaping uclamp constraints

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Message-Id: <20230820210640.585311-1-qyousef@layalina.io>
Date:   Sun, 20 Aug 2023 22:06:36 +0100
From:   Qais Yousef <qyousef@...alina.io>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Lukasz Luba <lukasz.luba@....com>,
        Qais Yousef <qyousef@...alina.io>
Subject: [PATCH 0/4] Fix dvfs_headroom escaping uclamp constraints

DVFS headroom, or map_util_perf(), is applied after uclamp restrictions are
applied in effective_cpu_util(). This will lead to two problems for uclamp:

	1. If util < uclamp_min, we'll run faster than uclamp_min. For example
	   util = 50, uclamp_min = 100. map_util_perf() = 125 instead of 100.

	2. If util > uclamp_max, we'll run faster than uclamp_max. For example
	   util = 900, uclamp_max = 800, map_util_perf() = 1000.

First patch rename the function to apply_dvfs_headroom() to reflect what it
really does. It is not really mapping util, but provides some headroom for the
util to grow. Provide a documentation for the function too.

Second patch is the actual fix.

Third patch moves apply_dvfs_headroom() to sched.h as there are no longer
users outside the scheduler.

Fourth patch is an RFC to redefine what the headroom means for RT, DL and IRQ
pressures.

Thanks!

--
Qais Yousef

Qais Yousef (4):
  sched: cpufreq: Rename map_util_perf to apply_dvfs_headroom
  sched: cpufreq: Fix apply_dvfs_headroom() escaping uclamp constraints
  sched: cpufreq: Move apply_dvfs_headroom() to sched.h
  sched: cpufreq: Apply DVFS headroom to CFS only

 include/linux/energy_model.h     |  1 -
 include/linux/sched/cpufreq.h    |  5 -----
 kernel/sched/core.c              |  7 +++++--
 kernel/sched/cpufreq_schedutil.c |  5 ++---
 kernel/sched/sched.h             | 24 ++++++++++++++++++++++++
 5 files changed, 31 insertions(+), 11 deletions(-)

-- 
2.34.1