linux-kernel - Re: [PATCH] sched/rt: Add a new sysctl to control uclamp_util

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Message-ID: <20200108134448.GG2844@hirez.programming.kicks-ass.net>
Date:   Wed, 8 Jan 2020 14:44:48 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        valentin.schneider@....com, qperret@...gle.com,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] sched/rt: Add a new sysctl to control uclamp_util_min

On Fri, Dec 20, 2019 at 04:48:38PM +0000, Qais Yousef wrote:
> RT tasks by default try to run at the highest capacity/performance
> level. When uclamp is selected this default behavior is retained by
> enforcing the uclamp_util_min of the RT tasks to be
> uclamp_none(UCLAMP_MAX), which is SCHED_CAPACITY_SCALE; the maximum
> value.
> 
> See commit 1a00d999971c ("sched/uclamp: Set default clamps for RT tasks").
> 
> On battery powered devices, this default behavior could consume more
> power, and it is desired to be able to tune it down. While uclamp allows
> tuning this by changing the uclamp_util_min of the individual tasks, but
> this is cumbersome and error prone.
> 
> To control the default behavior globally by system admins and device
> integrators, introduce the new sysctl_sched_rt_uclamp_util_min to
> change the default uclamp_util_min value of the RT tasks.
> 
> Whenever the new default changes, it'd be applied on the next wakeup of
> the RT task, assuming that it still uses the system default value and
> not a user applied one.

This is because these RT tasks are not in a cgroup or not affected by
cgroup settings? I feel the justification is a little thin here.

> If the uclamp_util_min of an RT task is 0, then the RT utilization of
> the rq is used to drive the frequency selection in schedutil for RT
> tasks.

Did cpu_uclamp_write() forget to check for input<0 ?