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

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Message-ID: <838f233e-fa4c-d5a3-9b50-69e2e121edda@arm.com>
Date:   Thu, 5 Dec 2019 15:03:51 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Valentin Schneider <valentin.schneider@....com>,
        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, 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 05/12/2019 11:49, Valentin Schneider wrote:
>  
> On 05/12/2019 09:24, Dietmar Eggemann wrote:
>> 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?
>>
> 
> Peter argued we should go for task attributes first, and then
> cgroup/taskgroups later on:
> 
> https://lore.kernel.org/lkml/20190905083127.GA2332@hirez.programming.kicks-ass.net/

OK, I went through this thread again. So Google or we have to provide
the missing per-taskgroup API via cpu controller's attributes (like for
uclamp) for the EAS usecase.

After reading:

https://lore.kernel.org/r/20190905114030.GL2349@hirez.programming.kicks-ass.net

IMHO the following mapping of the existing Android (binary)
latency_sensitive per-taskgroup flag makes sense:

latency_sensitive=1 -> latency_tolerance*[-20 .. -1] (less tolerant,
more sensitive)

latency_sensitive=0 -> latency_tolerance[0 .. 19] (more tolerant, less
sensitive)

Default value is 0 so not latency_sensitive.

* Since we use [-20 .. 19] as values for latency_tolerance we could name
it latency_nice. It's shorter ... ?