linux-kernel - Re: [PATCH v2 3/3] sched: Make uclamp changes depend on CAP_SYS

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Date:   Fri, 11 Jun 2021 13:08:15 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rickyiu@...gle.com, wvw@...gle.com,
        patrick.bellasi@...bug.net, xuewen.yan94@...il.com,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v2 3/3] sched: Make uclamp changes depend on CAP_SYS_NICE

Hi Qais,

On Friday 11 Jun 2021 at 13:48:20 (+0100), Qais Yousef wrote:
> On 06/10/21 15:13, Quentin Perret wrote:
> > There is currently nothing preventing tasks from changing their per-task
> > clamp values in anyway that they like. The rationale is probably that
> > system administrators are still able to limit those clamps thanks to the
> > cgroup interface. However, this causes pain in a system where both
> > per-task and per-cgroup clamp values are expected to be under the
> > control of core system components (as is the case for Android).
> > 
> > To fix this, let's require CAP_SYS_NICE to increase per-task clamp
> > values. This allows unprivileged tasks to lower their requests, but not
> > increase them, which is consistent with the existing behaviour for nice
> > values.
> 
> Hmmm. I'm not in favour of this.
> 
> So uclamp is a performance and power management mechanism, it has no impact on
> fairness AFAICT, so it being a privileged operation doesn't make sense.
> 
> We had a thought about this in the past and we didn't think there's any harm if
> a task (app) wants to self manage. Yes a task could ask to run at max
> performance and waste power, but anyone can generate a busy loop and waste
> power too.
> 
> Now that doesn't mean your use case is not valid. I agree if there's a system
> wide framework that wants to explicitly manage performance and power of tasks
> via uclamp, then we can end up with 2 layers of controls overriding each
> others.

Right, that's the main issue. Also, the reality is that most of time the
'right' clamps are platform-dependent, so most userspace apps are simply
not equipped to decide what their own clamps should be.

> Would it make more sense to have a procfs/sysfs flag that is disabled by
> default that allows sys-admin to enforce a privileged uclamp access?
> 
> Something like
> 
> 	/proc/sys/kernel/sched_uclamp_privileged

Hmm, dunno, I'm not aware of anything else having a behaviour like that,
so that feels a bit odd.

> I think both usage scenarios are valid and giving sys-admins the power to
> enforce a behavior makes more sense for me.

Yes, I wouldn't mind something like that in general. I originally wanted
to suggest introducing a dedicated capability for uclamp, but that felt
a bit overkill. Now if others think this should be the way to go I'm
happy to go implement it.

Thanks,
Quentin