linux-kernel - Re: [PATCH v3 2/2] Documentation/sysctl: Document uclamp sysctl knobs

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Date:   Tue, 28 Apr 2020 10:43:29 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Qais Yousef <qais.yousef@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] Documentation/sysctl: Document uclamp sysctl knobs

Hi--

I have a few corrections for you below:

On 4/28/20 9:41 AM, Qais Yousef wrote:
> Uclamp exposes 3 sysctl knobs:
> 
> 	* sched_util_clamp_min
> 	* sched_util_clamp_max
> 	* sched_util_clamp_min_rt_default
> 
> Document them in sysctl/kernel.rst.
> 
> Signed-off-by: Qais Yousef <qais.yousef@....com>
> CC: Jonathan Corbet <corbet@....net>
> CC: Juri Lelli <juri.lelli@...hat.com>
> CC: Vincent Guittot <vincent.guittot@...aro.org>
> CC: Dietmar Eggemann <dietmar.eggemann@....com>
> CC: Steven Rostedt <rostedt@...dmis.org>
> CC: Ben Segall <bsegall@...gle.com>
> CC: Mel Gorman <mgorman@...e.de>
> CC: Luis Chamberlain <mcgrof@...nel.org>
> CC: Kees Cook <keescook@...omium.org>
> CC: Iurii Zaikin <yzaikin@...gle.com>
> CC: Quentin Perret <qperret@...gle.com>
> CC: Valentin Schneider <valentin.schneider@....com>
> CC: Patrick Bellasi <patrick.bellasi@...bug.net>
> CC: Pavan Kondeti <pkondeti@...eaurora.org>
> CC: linux-doc@...r.kernel.org
> CC: linux-kernel@...r.kernel.org
> CC: linux-fsdevel@...r.kernel.org
> ---
>  Documentation/admin-guide/sysctl/kernel.rst | 48 +++++++++++++++++++++
>  1 file changed, 48 insertions(+)
> 
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index 0d427fd10941..e7255f71493c 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -940,6 +940,54 @@ Enables/disables scheduler statistics. Enabling this feature
>  incurs a small amount of overhead in the scheduler but is
>  useful for debugging and performance tuning.
>  
> +sched_util_clamp_min:
> +=====================
> +
> +Max allowed *minimum* utilization.
> +
> +Default value is SCHED_CAPACITY_SCALE (1024), which is the maximum possible
> +value.
> +
> +It means that any requested uclamp.min value cannot be greater than
> +sched_util_clamp_min, ie: it is restricted to the range

                         i.e., it is

> +[0:sched_util_clamp_min].
> +
> +sched_util_clamp_max:
> +=====================
> +
> +Max allowed *maximum* utilization.
> +
> +Default value is SCHED_CAPACITY_SCALE (1024), which is the maximum possible
> +value.
> +
> +It means that any requested uclamp.max value cannot be greater than
> +sched_util_clamp_max, ie: it is restricted to the range

                         i.e., it is

> +[0:sched_util_clamp_max].
> +
> +sched_util_clamp_min_rt_default:
> +================================
> +
> +By default Linux is tuned for performance. Which means that RT tasks always run
> +at the highest frequency and most capable (highest capacity) CPU (in
> +heterogeneous systems).
> +
> +Uclamp achieves this by setting the requested uclamp.min of all RT tasks to
> +SCHED_CAPACITY_SCALE (1024) by default. Which effectively boosts the tasks to

                               by default, which

> +run at the highest frequency and bias them to run on the biggest CPU.

                                    biases them

> +
> +This knob allows admins to change the default behavior when uclamp is being
> +used. In battery powered devices particularly, running at the maximum
> +capacity and frequency will increase energy consumption and shorten the battery
> +life.
> +
> +This knob is only effective for RT tasks which the user hasn't modified their
> +requested uclamp.min value via sched_setattr() syscall.
> +
> +This knob will not escape the constraint imposed by sched_util_clamp_min
> +defined above.
> +
> +Any modification is applied lazily on the next opportunity the scheduler needs
> +to calculate the effective value of uclamp.min of the task.
>  
>  seccomp
>  =======
> 

thanks.
-- 
~Randy