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From:   Joel Fernandes <joel@joelfernandes.org>
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Subject: Re: [PATCH v7 6/9] sched/fair: Add sched group latency support
Date:   Fri, 4 Nov 2022 09:13:10 -0400
Message-Id: <B7E91F49-3053-44E7-A515-8647A4C93D2D@joelfernandes.org>
References: <20221104112138.hlnwjmw2ls4gswuo@airbuntu>
Cc:     Vincent Guittot <vincent.guittot@linaro.org>, 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
In-Reply-To: <20221104112138.hlnwjmw2ls4gswuo@airbuntu>
To:     Qais Yousef <qyousef@layalina.io>
Precedence: bulk


> On Nov 4, 2022, at 7:21 AM, Qais Yousef <qyousef@layalina.io> wrote:
>=20
> =EF=BB=BFOn 11/03/22 18:02, Vincent Guittot wrote:
>>> On Thu, 3 Nov 2022 at 15:27, Qais Yousef <qyousef@layalina.io> wrote:
>>>=20
>>> On 11/03/22 09:46, Vincent Guittot wrote:
>>>> On Tue, 1 Nov 2022 at 20:28, Qais Yousef <qyousef@layalina.io> wrote:
>>>>>=20
>>>>> 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 entitie=
s
>>>>>> still have the default latency offset value.
>>>>>>=20
>>>>>> Add a latency.nice field in cpu cgroup controller to set the latency
>>>>>> priority of the group similarly to sched_setattr(). The latency prior=
ity
>>>>>> is then used to set the offset of the sched_entities of the group.
>>>>>>=20
>>>>>> 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(+)
>>>>>>=20
>>>>>> 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 utilizat=
ion
>>>>>>         value is used to clamp the task specific maximum utilization c=
lamp.
>>>>>>=20
>>>>>> +  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 th=
e
>>>>>> +     same values used by sched_setattr(2).
>>>>>=20
>>>>> I'm still not sure about this [1].
>>>>=20
>>>> I'm still not sure about what you are trying to say here ...
>>>>=20
>>>> This is about setting a latency nice prio to a group level.
>>>>=20
>>>>>=20
>>>>> In some scenarios we'd like to get the effective latency_nice of the t=
ask. How
>>>>> will the task inherit the cgroup value or be impacted by it?
>>>>>=20
>>>>> For example if there are tasks that belong to a latency sensitive cgro=
up; and
>>>>> I'd like to skip some searches in EAS to improve that latency sensitiv=
ity - how
>>>>> would I extract this info in EAS path given these tasks are using defa=
ult
>>>>> latency_nice value? And if should happen if their latency_nice is set t=
o
>>>>> something else other than default?
>>>>>=20
>>>>> [1] https://lore.kernel.org/lkml/20221012160734.hrkb5jcjdq7r23pr@wubun=
tu/
>>>>=20
>>>> Hmm so you are speaking about something that is not part of the patch.
>>>> Let focus on the patchset for now
>>>=20
>>> I am focusing on this patchset. Isn't this an essential part of the desi=
gn?
>>> Once the interface is out we can't change it. As it stands, I can't see h=
ow it
>>=20
>> So, are you speaking about the interface i.e. setting a value between [-2=
0:19]
>=20
> About how the cgroup and per task interface interact.
>=20
> How to get the effective value of latency_nice for a task that belongs to
> a hierarchy?
>=20
> If I have a task that has p->latency_nice =3D 20 but it belongs to a cgrou=
p that
> has tg->cpu.latency.nice =3D -19

Just for the task placement signal, One way is to go through the se hierarch=
y till the root and get the minimum. Then make that the effective value. So I=
n your example that would make it -19 so prefer idle =3D 1.  We should need a=
 Boolean signal. Not pretty but not the end of the world imho.

> And I want to use this interface in EAS; how should I interpret these valu=
es?
> How should I walk up the hierarchy and decide the _effective_ latency_nice=

> value

Ah, using a min function?

Joel


>>=20
> Thanks
>=20
> --
> Qais Yousef
>=20
>>=20
>>>=20
>>>=20
>>> Thanks
>>>=20
>>> --
>>> Qais Yousef