Re: [PATCH v7 6/9] sched/fair: Add sched group latency support

[Vincent Guittot]

On Fri, 4 Nov 2022 at 16:12, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, Nov 4, 2022 at 3:03 PM Vincent Guittot
> <vincent.guittot@xxxxxxxxxx> wrote:
> [...]
> > > > > > > On 11/03/22 09:46, Vincent Guittot wrote:
> > > > > > > > On Tue, 1 Nov 2022 at 20:28, Qais Yousef <qyousef@xxxxxxxxxxx> wrote:
> > > > > > > > >
> > > > > > > > > On 10/28/22 11:34, Vincent Guittot wrote:
> > > > > > > > > > Task can set its latency priority with sched_setattr(), which is then used
> > > > > > > > > > to set the latency offset of its sched_enity, 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@xxxxxxxxxx>
> > > > > > > > > > ---
> > > > > > > > > >  Documentation/admin-guide/cgroup-v2.rst |  8 ++++
> > > > > > > > > >  kernel/sched/core.c                     | 52 +++++++++++++++++++++++++
> > > > > > > > > >  kernel/sched/fair.c                     | 33 ++++++++++++++++
> > > > > > > > > >  kernel/sched/sched.h                    |  4 ++
> > > > > > > > > >  4 files changed, 97 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
> [...]
> > > > > > > >
> > > > > > > > Hmm so you are speaking about something that is not part of the patch.
> > > > > > > > Let focus on the patchset for now
> > > > > > >
> > > > > > > I am focusing on this patchset. Isn't this an essential part of the design?
> > > > > > > Once the interface is out we can't change it. As it stands, I can't see how it
> > > > > >
> > > > > > So, are you speaking about the interface i.e. setting a value between [-20:19]
> > > > >
> > > > > About how the cgroup and per task interface interact.
> > > > >
> > > > > How to get the effective value of latency_nice for a task that belongs to
> > > > > a hierarchy?
> > > >
> > > > At the common parents level of the 2 entities that you want to compare
> > > > and root level if there no other entity to compare with
> > > >
> > > > >
> > > > > If I have a task that has p->latency_nice = 20 but it belongs to a cgroup that
> > > > > has tg->cpu.latency.nice = -19
> > > >
> > > > according to what i said above, latency_nice = 20 inside the group and
> > > > -19 when comparing at the parent level
> > > >
> > > > >
> > > > > And I want to use this interface in EAS; how should I interpret these values?
> > > > > How should I walk up the hierarchy and decide the _effective_ latency_nice
> > > > > value?
> > > >
> > > > The current use of latency_nice doesn't need to walk the hierarchy
> > > > because it applies at each scheduling level so the childs
> > > > automatically follow parents' latency.
> > >
> > > Not really, I don't see how that will work that way in the wake up path. The
> > > wake up path (EAS in particular) does not walk through CPU controller group
> > > hierarchy from top level, it only cares about cpuset/affinities and the
> > > "effective" values of tasks.
> >
> > I was explaining the current use of latency_ni i.e. in this patchset,
> > I'm not speaking about what should be done for other case like EAS
>
> That's fine, but you did mention the negative value of latency_nice
> used to mark that a task prefers idle CPU so we should finish that
> discussion :-D.  Since that will be one of the potential users of this
> patchset.

We should finish reviewing this patchset 1st. Then, we can discuss if
we should use -19, <0 or something else. This doesn't add any value to
this patchset IMO

>
> > In fact, it's exactly what I explained few lines above :
> > "> > At the common parents level of the 2 entities that you want to compare
> > > > and root level if there no other entity to compare with"
>
> Yes this is a different usecase, but having more real world use cases
> will add more purpose to the patchset.

As long as the policy is defined and I think it is defined, we are
fine and it's not the purpose of discussing implementation details of
potential other use. Once this patchset is merged, I will be more than
happy to prepare another one to make use of latency_nice in EAS and we
can discuss further based on it.


>
> I also want to add -- for ChromeOS, Youssef tried it and the temporary
> boost that latency_nice gives is not enough for latency-sensitive
> workloads (like the main thread of a ChromeOS web page which is both
> CPU bound and handles latency-sensitive user input). So we are also
> exploring other ways. However, I have no issue with the patchset if it
> helps Android and would love to review further.
>
> Thanks.