Re: [PATCH v2 10/12] sched/core: uclamp: use TG's clamps to restrict Task's clamps

[Suren Baghdasaryan]

Hi Patrick. Thanks for the explanation and links. No more questions
from me on this one :)

On Tue, Jul 24, 2018 at 2:56 AM, Patrick Bellasi
<patrick.bellasi@xxxxxxx> wrote:
> On 23-Jul 10:11, Suren Baghdasaryan wrote:
>> On Mon, Jul 23, 2018 at 8:40 AM, Patrick Bellasi
>> <patrick.bellasi@xxxxxxx> wrote:
>> > On 21-Jul 20:05, Suren Baghdasaryan wrote:
>> >> On Mon, Jul 16, 2018 at 1:29 AM, Patrick Bellasi
>
> [...]
>
>> >> So to satisfy both TG and syscall requirements I think you would
>> >> need to choose the largest value for UCLAMP_MIN and the smallest one
>> >> for UCLAMP_MAX, meaning the most boosted and most clamped range.
>> >> Current implementation choses the least boosted value, so
>> >> effectively one of the UCLAMP_MIN requirements (either from TG or
>> >> from syscall) are being ignored...  Could you please clarify why
>> >> this choice is made?
>> >
>> > The TG values are always used to specify a _restriction_ on
>> > task-specific values.
>> >
>> > Thus, if you look or example at the CPU mask for a task, you can have
>> > a task with affinity to CPUs 0-1, currently running on a cgroup with
>> > cpuset.cpus=0... then the task can run only on CPU 0 (althought its
>> > affinity includes CPU1 too).
>> >
>> > Same we do here: if a task has util_min=10, but it's running in a
>> > cgroup with cpu.util_min=0, then it will not be boosted.
>> >
>> > IOW, this allows to implement a "nice" policy at task level, where a
>> > task (via syscall) can decide to be less boosted with respect to its
>> > group but never more boosted. The same task can also decide to be more
>> > clamped, but not less clamped then its current group.
>> >
>>
>> The fact that boost means "at least this much" to me seems like we can
>> safely choose higher CPU bandwidth (as long as it's lower than
>> UCLAMP_MAX)
>
> I understand your view point, which actually is matching my first
> implementation for util_min aggregation:
>
>    https://lore.kernel.org/lkml/20180409165615.2326-5-patrick.bellasi@xxxxxxx/
>
>
>> but from your description sounds like TG's UCLAMP_MIN means "at most
>> this much boost" and it's not safe to use CPU bandwidth higher than
>> TG's UCLAMP_MIN.
>
> Indeed, after this discussion with Tejun:
>
>    https://lore.kernel.org/lkml/20180409222417.GK3126663@xxxxxxxxxxxxxxxxxxxxxxxxxxx/
>
> I've convinced myself that for the cgroup interface we have to got for
> a "restrictive" interface where a parent value must set the upper
> bound for all its descendants values. AFAIU, that's one of the basic
> principles of the "delegation model" implemented by cgroups and the
> common behavior implemented by all controllers.
>
>> So instead of specifying min CPU bandwidth for a task it specifies
>> the max allowed boost. Seems like a discrepancy to me but maybe
>> there are compelling usecases when this behavior is necessary?
>
> I don't think it's strictly related to use-cases, you can always
> describe a give use-case in one model or the other.  It all depends on
> how you configure your hierarchy and where you place your tasks.
>
> For our Android use cases, we are still happy to say that all tasks of
> a CGroup can be boosted up to a certain value and then we can either:
> - don't configure tasks: and thus get the CG defined boost
> - configure a task: and explicitly give back what we don't need
>
> This model works quite well with containers, where the parent want to
> precisely control how much resources are (eventually) usable by a
> given container.
>
>> In that case would be good to spell them out to explain why this
>> choice is made.
>
> Yes, well... if I understand it correctly is really just the
> recommended way cgroups must be used to re-partition resources.
>
> I'll try to better explain this behavior in the changelog for this
> patch.
>
> [...]
>
> Best,
> Patrick
>
> --
> #include <best/regards.h>
>
> Patrick Bellasi