Re: [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping for RT tasks

Dietmar Eggemann
2018-08-16T10:34:14Z

On 08/13/2018 05:01 PM, Patrick Bellasi wrote:
> On 13-Aug 16:06, Vincent Guittot wrote:
>> On Mon, 13 Aug 2018 at 14:49, Patrick Bellasi <patrick.bellasi@arm.com> wrote:
>>> On 13-Aug 14:07, Vincent Guittot wrote:
>>>> On Mon, 13 Aug 2018 at 12:12, Patrick Bellasi <patrick.bellasi@arm.com> wrote:
>
> [...]
>
>>> Yes I agree that the current behavior is not completely clean... still
>>> the question is: do you reckon the problem I depicted above, i.e. RT
>>> workloads eclipsing the min_util required by lower priority classes?
>>
>> As said above, I don't think that there is a problem that is specific
>> to cross class scheduling that can't also happen in the same class.
>>
>> Regarding your example:
>> task TA util=40% with uclamp_min 50%
>> task TB util=10% with uclamp_min 0%
>>
>> If TA and TB are cfs, util=50% and it doesn't seem to be a problem
>> whereas TB will steal some bandwidth to TA and delay it (and i even
>> don't speak about the impact of the nice priority of TB)
>> If TA is cfs and TB is rt, Why util=50% is now a problem for TA ?
>
> You right, in the current implementation, where we _do not_
> distinguish among scheduling classes it's not possible to get a
> reasonable implementation of a per sched class clamping.
>
>>> To a certain extend I see this problem similar to the rt/dl/irq pressure
>>> in defining cpu_capacity, isn't it?
>
> However, I still think that higher priority classes eclipsing the
> clamping of lower priority classes can still be a problem.
>
> In your example above, the main difference between TA and TB being on
> the same class or different classes is that in the second case TB
> is granted to always preempt TA. We can end up with a non boosted RT
> task consuming all the boosted bandwidth required by a CFS task.
>
> This does not happen, apart maybe for the corner case of really
> different nice values, if the tasks are both CFS, since the fair
> scheduler will grant some progress for both of them.
>
> Thus, given the current implementation, I think it makes sense to drop
> the UCLAMP_SCHED_CLASS policy and stick with a more clean and
> consistent design.

I agree with everything said in this thread so far.
So in case you skip UCLAMP_SCHED_CLASS [(B) combine the clamped
utilizations] in v4, you will only provide [A) clamp the combined
utilization]?

I assume that we don't have to guard the util clamping for rt tasks
behind a disabled by default sched feature because all runnable rt tasks
will have util_min = SCHED_CAPACITY_SCALE by default?

> I'll then see if it makes sense to add a dedicated patch on top of the
> series to add a proper per-class clamp tracking.

I assume if you introduce this per-class clamping you will switch to use
the UCLAMP_SCHED_CLASS approach?