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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Fri, 4 Nov 2022 15:12:36 +0000
Message-ID: <CAEXW_YSHC0ahXKdj8-U5DZ9=dAuoomfMSs7m687zBZrdNq-uxg@mail.gmail.com>
Subject: Re: [PATCH v7 6/9] sched/fair: Add sched group latency support
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Qais Yousef <qyousef@layalina.io>, mingo@redhat.com,
        peterz@infradead.org, juri.lelli@redhat.com,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
        linux-kernel@vger.kernel.org, parth@linux.ibm.com,
        qais.yousef@arm.com, chris.hyser@oracle.com,
        patrick.bellasi@matbug.net, David.Laight@aculab.com,
        pjt@google.com, pavel@ucw.cz, tj@kernel.org, qperret@google.com,
        tim.c.chen@linux.intel.com, joshdon@google.com, timj@gnu.org,
        kprateek.nayak@amd.com, yu.c.chen@intel.com,
        youssefesmat@chromium.org
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On Fri, Nov 4, 2022 at 3:03 PM Vincent Guittot
<vincent.guittot@linaro.org> wrote:
[...]
> > > > > > On 11/03/22 09:46, Vincent Guittot wrote:
> > > > > > > On Tue, 1 Nov 2022 at 20:28, Qais Yousef <qyousef@layalina.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@linaro.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
[...]
> > > > > > >
> > > > > > > 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.

> 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.

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.