Re: [PATCH v4 14/16] sched/core: uclamp: request CAP_SYS_ADMIN by default

From: Patrick Bellasi <patrick.bellasi@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>,
	linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
	Ingo Molnar <mingo@redhat.com>, Tejun Heo <tj@kernel.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Paul Turner <pjt@google.com>,
	Quentin Perret <quentin.perret@arm.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Todd Kjos <tkjos@google.com>, Joel Fernandes <joelaf@google.com>,
	Steve Muckle <smuckle@google.com>,
	Suren Baghdasaryan <surenb@google.com>
Subject: Re: [PATCH v4 14/16] sched/core: uclamp: request CAP_SYS_ADMIN by default
Date: Fri, 14 Sep 2018 15:07:32 +0100	[thread overview]
Message-ID: <20180914140732.GR1413@e110439-lin> (raw)
In-Reply-To: <20180914111003.GC24082@hirez.programming.kicks-ass.net>

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