linux-kernel - Re: [RFC 0/3] Introduce per-task latency

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Message-ID: <450fc7e2-49d5-d809-281f-7d9a99d3e530@arm.com>
Date:   Thu, 5 Dec 2019 10:24:33 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Parth Shah <parth@...ux.ibm.com>, linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
        patrick.bellasi@...bug.net, valentin.schneider@....com,
        qais.yousef@....com, pavel@....cz, dhaval.giani@...cle.com,
        qperret@...rret.net, David.Laight@...LAB.COM,
        morten.rasmussen@....com, pjt@...gle.com, tj@...nel.org,
        viresh.kumar@...aro.org, rafael.j.wysocki@...el.com,
        daniel.lezcano@...aro.org
Subject: Re: [RFC 0/3] Introduce per-task latency_tolerance for scheduler
 hints

On 25/11/2019 10:46, Parth Shah wrote:
> This patch series is based on the discussion started as the "Usecases for
> the per-task latency-nice attribute"[1]
> 
> This patch series introduces a new per-task attribute latency_tolerance to
> provide the scheduler hints about the latency requirements of the task.

I forgot but is there a chance to have this as a per-taskgroup attribute
as well?

> Latency_tolerance is a ranged attribute of a task with the value ranging
> from [-20, 19] both inclusive which makes it align with the task nice
> value.
> 
> The value should provide scheduler hints about the relative latency
> requirements of tasks, meaning the task with "latency_tolerance = -20"
> should have lower latency than compared to those tasks with higher values.
> Similarly a task with "latency_tolerance = 19" can have higher latency and
> hence such tasks may bot care much about the latency numbers.
> 
> The default value is set to 0. The usecases defined in [1] can use this
> range of [-20, 19] for latency_tolerance for the specific purpose. This
> patch does not define any use cases for such attribute so that any change
> in naming or range does not affect much to the other (future) patches using
> this. The actual use of latency_tolerance during task wakeup and
> load-balancing is yet to be coded for each of those usecases.

This can definitely be useful for Android/EAS by replacing the current
proprietary solution in android-google-common android-5.4:

commit 760b82c9b88d ("ANDROID: sched/fair: Bias EAS placement for latency")
commit c28f9d3945f1 ("ANDROID: sched/core: Add a latency-sensitive flag
to uclamp")

which links to usercase 6 (EAS) in [1].