linux-kernel - Re: [PATCH v4 14/16] sched/core: uclamp: request CAP_SYS

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Message-ID: <20180914140732.GR1413@e110439-lin>
Date:   Fri, 14 Sep 2018 15:07:32 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Juri Lelli <juri.lelli@...hat.com>, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v4 14/16] sched/core: uclamp: request CAP_SYS_ADMIN by
 default

On 14-Sep 13:10, Peter Zijlstra wrote:
> On Thu, Sep 06, 2018 at 03:40:53PM +0100, Patrick Bellasi wrote:
> > 1) _I think_ we don't want to depend on capable(CAP_SYS_NICE) but
> >    instead on capable(CAP_SYS_ADMIN)
> > 
> >    Does that make sense ?
> 
> Neither of them really makes sense to me.
> 
> The max clamp makes a task 'consume' less and you should always be able
> to reduce yourself.
> 
> The min clamp doesn't avoid while(1); and is therefore also not a
> problem.
> 
> So I think setting clamps on a task should not be subject to additional
> capabilities.
> 
> Now, of course, there is a problem of clamp resources, which are
> limited. Consuming those _is_ a problem.

Right, that problem could be solved if we convince ourself that the
quantization approach proposed in:

   [PATCH v4 15/16] sched/core: uclamp: add clamp group discretization support
   https://lore.kernel.org/lkml/20180828135324.21976-16-patrick.bellasi@arm.com/

could make sense and specifically, the other limitation it imposes
(i.e. the quantizaiton) is within reasonable rounding control errors/

> I think the problem here is that the two are conflated in the very same
> interface.
> 
> Would it make sense to move the available clamp values out to some sysfs
> interface like thing and guard that with a capability, while keeping the
> task interface unprivilidged?

You mean something like:

   $ cat /proc/sys/kernel/sched_uclamp_min_utils
   0 10 20 ... 100

to notify users about the set of clamp values which are available ?

> Another thing that has me 'worried' about this interface is the direct
> tie to CPU capacity (not that I have a better suggestion). But it does
> raise the point of how userspace is going to discover the relevant
> values of the platform.

This point worries me too, and that's what I think is addressed in a
sane way in:

   [PATCH v4 13/16] sched/core: uclamp: use percentage clamp values
   https://lore.kernel.org/lkml/20180828135324.21976-14-patrick.bellasi@arm.com/

IMHO percentages are a reasonably safe and generic API to expose to
user-space. Don't you think this should address your concern above ?

-- 
#include <best/regards.h>

Patrick Bellasi