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

[Joel Fernandes]

> On Nov 4, 2022, at 6:37 AM, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
> 
> On Fri, 4 Nov 2022 at 11:15, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>> 
>>> On Thu, Nov 3, 2022 at 5:03 PM Vincent Guittot
>>> <vincent.guittot@xxxxxxxxxx> wrote:
>>> 
>>> On Thu, 3 Nov 2022 at 15:27, Qais Yousef <qyousef@xxxxxxxxxxx> 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
>>>>>>> +     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  ?

Ah and forgot to reply about negative.

Maybe, but it’s still a horrible overload of the meaning of the value. I am not terribly against choosing negative value if there is consensus among everyone. Qais?

- Joel


> 
>> 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