linux-kernel - Re: [PATCH v4 2/2] Documentation/sysctl: Document uclamp sysctl knobs

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Date:   Sun, 03 May 2020 19:45:23 +0200
From:   Patrick Bellasi <derkling@...il.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        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>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] Documentation/sysctl: Document uclamp sysctl knobs


Hi Qais,

On Fri, May 01, 2020 at 13:49:27 +0200, Qais Yousef <qais.yousef@....com> wrote...

[...]

> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index 0d427fd10941..521c18ce3d92 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
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Mmm... I feel one of the two is an implementation detail which should
probably not be exposed?

The user perhaps needs to know the value (1024) but we don't need to
expose the internal representation.


> +value.
> +
> +It means that any requested uclamp.min value cannot be greater than
> +sched_util_clamp_min, i.e., it is restricted to the range
> +[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, i.e., it is restricted to the range
> +[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
> +run at the highest frequency and biases them to run on the biggest CPU.
> +
> +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.

Perhaps it's worth to specify that this value is going to be clamped by
the values above? Otherwise it's a bit ambiguous to know what happen
when it's bigger than schedu_util_clamp_min.

> +Any modification is applied lazily on the next opportunity the scheduler needs
> +to calculate the effective value of uclamp.min of the task.
                    ^^^^^^^^^

This is also an implementation detail, I would remove it.

>  
>  seccomp
>  =======


Best,
Patrick