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

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Message-ID: <20200430100311.hcamfff363hl4bjk@e107158-lin>
Date:   Thu, 30 Apr 2020 11:03:13 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Tao Zhou <ouwen210@...mail.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>,
        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

On 04/30/20 13:06, Tao Zhou wrote:
> Hi,
> 
> On Tue, Apr 28, 2020 at 05:41:34PM +0100, 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
>                                                           ^^^^^^^
> 
> Seems that 'greater' should be 'smaller'.
> And the range is [sched_util_clamp_min:SCHED_CAPACITY_SCALE]
> Uclamp request should not below this system value.
> Or I am totally wrong for memory leak.

No the range is [0:sched_util_clamp_min].

It is what it is implemented by this condition:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/sched/core.c?h=v5.7-rc3#n913

So if the requested uclamp.min value is greater than sched_util_clamp_min, we
return sched_util_clamp_min.

Thanks

--
Qais Yousef

> 
> Thank you
> 
> > +sched_util_clamp_min, ie: 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, ie: 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 bias 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.
> > +
> > +Any modification is applied lazily on the next opportunity the scheduler needs
> > +to calculate the effective value of uclamp.min of the task.
> >  
> >  seccomp
> >  =======
> > -- 
> > 2.17.1
> >