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

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Message-ID: <877dxik4ob.derkling@matbug.com>
Date:   Mon, 11 May 2020 15:00:04 +0200
From:   Patrick Bellasi <patrick.bellasi@...bug.net>
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 Tue, May 05, 2020 at 16:56:37 +0200, Qais Yousef <qais.yousef@....com> wrote...

>> > +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.
>
> Hmm for me that sentence says exactly what you're asking for.
>
> So what you want is
>
> 	s/will not escape the constraint imposed by/will be clamped by/
>
> ?
>
> I'm not sure if this will help if the above is already ambiguous. Maybe if
> I explicitly say
>
> 	..will not escape the *range* constrained imposed by..
>
> sched_util_clamp_min is already defined as a range constraint, so hopefully it
> should hit the mark better now?

Right, that also can work.

>> 
>> > +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.
>
> The idea is that this value is not updated 'immediately'/synchronously. So
> currently RUNNING tasks will not see the effect, which could generate confusion
> when users trip over it. IMO giving an idea of how it's updated will help with
> expectation of the users. I doubt any will care, but I think it's an important
> behavior element that is worth conveying and documenting. I'd be happy to
> reword it if necessary.

Right, I agree on giving an hint on the lazy update. What I was pointing
out was mainly the reference to the 'effective' value. Maybe we can just
drop that word.

> I have this now
>
> """
>  984 This knob will not escape the range constraint imposed by sched_util_clamp_min
>  985 defined above.
>  986
>  987 For example if
>  988
>  989         sched_util_clamp_min_rt_default = 800
>  990         sched_util_clamp_min = 600
>  991
>  992 Then the boost will be clamped to 600 because 800 is outside of the permissible
>  993 range of [0:600]. This could happen for instance if a powersave mode will
>  994 restrict all boosts temporarily by modifying sched_util_clamp_min. As soon as
>  995 this restriction is lifted, the requested sched_util_clamp_min_rt_default
>  996 will take effect.
>  997
>  998 Any modification is applied lazily to currently running tasks and should be
>  999 visible by the next wakeup.
> """

That's better IMHO, would just slightly change the last sentence to:

       Any modification is applied lazily to tasks and is effective
       starting from their next wakeup.

Best,
Patrick