On 14-Sep 13:10, Peter Zijlstra wrote:

On Thu, Sep 06, 2018 at 03:40:53PM +0100, Patrick Bellasi wrote:

quoted

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/ (local)

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/ (local)

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