From: Quentin Perret <qperret@google.com>
To: Qais Yousef <qais.yousef@arm.com>
Cc: mingo@redhat.com, peterz@infradead.org,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
rickyiu@google.com, wvw@google.com, patrick.bellasi@matbug.net,
xuewen.yan94@gmail.com, linux-kernel@vger.kernel.org,
kernel-team@android.com
Subject: Re: [PATCH v2 3/3] sched: Make uclamp changes depend on CAP_SYS_NICE
Date: Fri, 11 Jun 2021 13:08:15 +0000 [thread overview]
Message-ID: <YMNgPyfiIaIIsjqq@google.com> (raw)
In-Reply-To: <20210611124820.ksydlg4ncw2xowd3@e107158-lin.cambridge.arm.com>
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
next prev parent reply other threads:[~2021-06-11 13:09 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-10 15:13 [PATCH v2 0/3] A few uclamp fixes Quentin Perret
2021-06-10 15:13 ` [PATCH v2 1/3] sched: Fix UCLAMP_FLAG_IDLE setting Quentin Perret
2021-06-10 19:05 ` Peter Zijlstra
2021-06-11 7:25 ` Quentin Perret
2021-06-17 15:27 ` Dietmar Eggemann
2021-06-21 10:57 ` Quentin Perret
2021-06-10 15:13 ` [PATCH v2 2/3] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS Quentin Perret
2021-06-10 19:15 ` Peter Zijlstra
2021-06-11 8:59 ` Quentin Perret
2021-06-11 9:07 ` Quentin Perret
2021-06-11 9:20 ` Peter Zijlstra
2021-06-10 15:13 ` [PATCH v2 3/3] sched: Make uclamp changes depend on CAP_SYS_NICE Quentin Perret
2021-06-11 12:48 ` Qais Yousef
2021-06-11 13:08 ` Quentin Perret [this message]
2021-06-11 13:26 ` Qais Yousef
2021-06-11 13:49 ` Quentin Perret
2021-06-11 14:17 ` Qais Yousef
2021-06-11 14:43 ` Quentin Perret
2021-06-14 15:03 ` Qais Yousef
2021-06-21 10:52 ` Quentin Perret
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