Date:   Fri, 4 Nov 2022 11:37:03 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Qais Yousef <qyousef@...alina.io>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, parth@...ux.ibm.com,
        qais.yousef@....com, chris.hyser@...cle.com,
        patrick.bellasi@...bug.net, David.Laight@...lab.com,
        pjt@...gle.com, pavel@....cz, tj@...nel.org, qperret@...gle.com,
        tim.c.chen@...ux.intel.com, joshdon@...gle.com, timj@....org,
        kprateek.nayak@....com, yu.c.chen@...el.com,
        youssefesmat@...omium.org
Subject: Re: [PATCH v7 6/9] sched/fair: Add sched group latency support

On Fri, 4 Nov 2022 at 11:15, Joel Fernandes <joel@...lfernandes.org> wrote:
>
> On Thu, Nov 3, 2022 at 5:03 PM Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
> >
> > On Thu, 3 Nov 2022 at 15:27, Qais Yousef <qyousef@...alina.io> wrote:
> > >
> > > On 11/03/22 09:46, Vincent Guittot wrote:
> > > > On Tue, 1 Nov 2022 at 20:28, Qais Yousef <qyousef@...alina.io> 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@...aro.org>
> > > > > > ---
> > > > > >  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
> > > > > > +     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'm still not sure about this [1].
> > > >
> > > > I'm still not sure about what you are trying to say here ...
> > > >
> > > > This is about setting a latency nice prio to a group level.
> > > >
> > > > >
> > > > > In some scenarios we'd like to get the effective latency_nice of the task. How
> > > > > will the task inherit the cgroup value or be impacted by it?
> > > > >
> > > > > For example if there are tasks that belong to a latency sensitive cgroup; and
> > > > > I'd like to skip some searches in EAS to improve that latency sensitivity - how
> > > > > would I extract this info in EAS path given these tasks are using default
> > > > > latency_nice value? And if should happen if their latency_nice is set to
> > > > > something else other than default?
> > > > >
> > > > > [1] https://lore.kernel.org/lkml/20221012160734.hrkb5jcjdq7r23pr@wubuntu/
> > > >
> > > > 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]
> >
> > > can be used to replace prefer_idle in cgroup as used in Android. I can't see
> > > how this could happen if we don't define how the task will inherit the cgroup
> > > value. If we can, mind elaborating how please?
> >
> > Or how to take into account the value set for a cgroup ?
> >
> > Regarding the behavior, the rule remains the same that a sched_entity
> > attached to a cgroup will not get more (latency in this case) than
> > what has been set for the group entity.
>
> I think the interface solves a different problem which is latency of
> task or cgroup wrt other group. Vincent, you are setting this for a
> “top app” group in android in your tests, and seeing improvement
> correct? AFAICS, this improvement comes because of lower latency

Yes Top app and display group

> during *same CPU* competition between different groups by juggling
> around the wakeup-preemption window -- which maybe is good for
> Android.
>
> OTOH, the “prefer idle” flag in android that Qais is referring to,
> will need a completely different method as I cannot see how a nice
> value can communicate that (that can complement Vincent's changes
> here). And it will need to have a per-task interface as well. We have

Why a negative latency_nice value condition can't be used ? or latency -20  ?

> something in ChromeOS as well, which is a proc knob and also
> out-of-tree patch for that [1]. Without [1] we fail Android CTS
> testing on a recent ARM64 ChromeOS device.
> [1] https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3884575
> The changelog in [1] also has a detailed description of the ChromeOS usecase.
>
> Qais, any other reason you can see why Vincent's change will not be a
> good thing for Android? Since you 1 CGroup for the whole user-facing
> app (top app), you can just set that to a low "latency_nice" and get
> better wake-up latency for that.
>
> (Side rant about latency and CFS -- IMHO a better long term solution
> for lower latency is to use RT but don't throttle -- rather demote. Or
> break CFS into multiple tiers, and apply demotion. This is in a way
> what Vincent is doing, as the task becomes more CPU bound'ish, he's
> taking away the latency boost. Vincent/Qais, somebody was working on
> the RT demotion vs throttling a while back, any idea on the latest on
> that?).
>
> thanks,
>
>  - Joel
>
>
> >
> > >
> > >
> > > Thanks
> > >
> > > --
> > > Qais Yousef

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