Re: [PATCH v5 5/7] sched/fair: Add sched group latency support

[Vincent Guittot <vincent.guittot@linaro.org>]

On Wed, 12 Oct 2022 at 16:22, Qais Yousef <qais.yousef@arm.com> wrote:
>
> On 09/25/22 16:39, Vincent Guittot wrote:
> > Task can set its latency priority with sched_setattr(), which is then used
> > to set the latency offset of its sched_entity, but sched group entities
> > still have the default latency offset value.
> >
> > Add a latency.nice field in cpu cgroup controller to set the latency
> > priority of the group similarly to sched_setattr(). The latency priority
> > is then used to set the offset of the sched_entities of the group.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> > ---
> >  Documentation/admin-guide/cgroup-v2.rst |  8 ++++
> >  kernel/sched/core.c                     | 53 +++++++++++++++++++++++++
> >  kernel/sched/fair.c                     | 33 +++++++++++++++
> >  kernel/sched/sched.h                    |  4 ++
> >  4 files changed, 98 insertions(+)
> >
> > diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> > index be4a77baf784..d8ae7e411f9c 100644
> > --- a/Documentation/admin-guide/cgroup-v2.rst
> > +++ b/Documentation/admin-guide/cgroup-v2.rst
> > @@ -1095,6 +1095,14 @@ All time durations are in microseconds.
> >          values similar to the sched_setattr(2). This maximum utilization
> >          value is used to clamp the task specific maximum utilization clamp.
> >
> > +  cpu.latency.nice
> > +     A read-write single value file which exists on non-root
> > +     cgroups.  The default is "0".
> > +
> > +     The nice value is in the range [-20, 19].
> > +
> > +     This interface file allows reading and setting latency using the
> > +     same values used by sched_setattr(2).
>
> I still don't understand how tasks will inherit the latency_nice value from
> cgroups they're attached to.

The behavior is the same as for sched_entity weight. The latency is
applied on the sched_entity of the group

>
> For example, in EAS path we operate at task level only. If the task's
> p->latency_nice = 0, but it belongs to a task group tg->latency_nice = -19;
> what should the task's latency_nice be in this case? If it's in a hierarchy,
> how would the effective value be calculated?
>
>
> Thanks
>
> --
> Qais Yousef