Messages in this thread | | | Date | Fri, 11 Jun 2021 14:26:53 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH v2 3/3] sched: Make uclamp changes depend on CAP_SYS_NICE |
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Hi Quentin
On 06/11/21 13:08, Quentin Perret wrote: > 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.
I'd argue this is true for both a framework or an app point of view. It depends on the application and how it would be used.
I can foresee for example and HTTP server wanting to use uclamp to guarantee a QoS target ie: X number of requests per second or a maximum of Y tail latency. The application can try to tune (calibrate) itself without having to have the whole system tuned or pumped on steroid.
Or a framework could manage this on behalf of the application. Both can use uclamp with a feedback loop to calibrate the perf requirement of the tasks to meet a given perf/power criteria.
If you want to do a static management, system framework would make more sense in this case, true.
> > > 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 /proc/sys/kernel/perf_event_paranoid falls into this category.
> > > 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.
Would be good to hear what others think for sure :)
Cheers
-- Qais Yousef
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