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

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Date:   Fri, 11 Jun 2021 13:48:20 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Quentin Perret <qperret@...gle.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

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.

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

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

Unless there's a real concern in terms of security/fairness that we missed?

Cheers

--
Qais Yousef