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Subject: Re: [RFC PATCH 1/9] sched,cgroup: Add interface for latency-nice
To:     Joel Fernandes <joel@joelfernandes.org>,
        Qais Yousef <qais.yousef@arm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Subhra Mazumdar <subhra.mazumdar@oracle.com>,
        linux-kernel@vger.kernel.org, mingo@redhat.com, tglx@linutronix.de,
        steven.sistare@oracle.com, dhaval.giani@oracle.com,
        daniel.lezcano@linaro.org, vincent.guittot@linaro.org,
        viresh.kumar@linaro.org, tim.c.chen@linux.intel.com,
        mgorman@techsingularity.net, parth@linux.ibm.com
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From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Message-ID: <730928f8-b48b-ea3a-149a-18932eb18c90@arm.com>
Date:   Thu, 16 Apr 2020 19:23:31 +0200
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Hi Joel,

On 16.04.20 02:02, Joel Fernandes wrote:
> On Thu, Sep 05, 2019 at 12:47:26PM +0100, Qais Yousef wrote:
>> On 09/05/19 13:30, Peter Zijlstra wrote:
>>> On Thu, Sep 05, 2019 at 12:13:47PM +0100, Qais Yousef wrote:
>>>> On 09/05/19 12:46, Peter Zijlstra wrote:
>>>
>>>>> This is important because we want to be able to bias towards less
>>>>> importance to (tail) latency as well as more importantance to (tail)
>>>>> latency.
>>>>>
>>>>> Specifically, Oracle wants to sacrifice (some) latency for throughput.
>>>>> Facebook OTOH seems to want to sacrifice (some) throughput for latency.
>>>>
>>>> Another use case I'm considering is using latency-nice to prefer an idle CPU if
>>>> latency-nice is set otherwise go for the most energy efficient CPU.
>>>>
>>>> Ie: sacrifice (some) energy for latency.
>>>>
>>>> The way I see interpreting latency-nice here as a binary switch. But
>>>> maybe we can use the range to select what (some) energy to sacrifice
>>>> mean here. Hmmm.
>>>
>>> It cannot be binary, per definition is must be ternary, that is, <0, ==0
>>> and >0 (or middle value if you're of that persuasion).
>>
>> I meant I want to use it as a binary.
>>
>>>
>>> In your case, I'm thinking you mean >0, we want to lower the latency.
>>
>> Yes. As long as there's an easy way to say: does this task care about latency
>> or not I'm good.
> 
> Qais, Peter, all,
> 
> For ChromeOS (my team), we are planning to use the upstream uclamp mechanism
> instead of the out-of-tree schedtune mechanism to provide EAS with the
> latency-sensitivity (binary/ternary) hint. ChromeOS is thankfully quite a bit
> upstream focussed :)
> 
> However, uclamp is missing an attribute to provide this biasing to EAS as we
> know.
> 
> What was the consensus on adding a per-task attribute to uclamp for providing
> this? Happy to collaborate on this front.

We're planning to have a session about this topic (latency-nice
attribute per task group) during the virtual Pisa OSPM summit
retis.sssup.it/ospm-summit in May this year.

There are two presentations/discussions planned:

"Introducing Latency Nice for Scheduler Hints and Optimizing Scheduler
Task Wakeup" and "The latency nice use case for Energy-Aware-Scheduling
(EAS) in Android Common Kernel (ACK)"

We'll probably merge those two into one presentation/discussion.

So far we have Parth's per-task implementation

https://lore.kernel.org/lkml/20200228090755.22829-1-parth@linux.ibm.com

What's missing is the per-taskgroup implementation, at least from the
standpoint of ACK.

The (mainline) EAS use-case for latency nice is already in ACK
(android-5.4):

https://android.googlesource.com/kernel/common/+/760b82c9b88d2c8125abfc5f732cc3cd460b2a54