linux-kernel - Re: [Discussion v2] Usecases for the per-task latency-nice attribute

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Message-Id: <376e1a37-8f7d-3cbc-9d9d-dd349afb9b3b@linux.ibm.com>
Date:   Thu, 24 Oct 2019 13:49:34 +0530
From:   Parth Shah <parth@...ux.ibm.com>
To:     linux-kernel@...r.kernel.org, patrick.bellasi@...bug.net,
        tim.c.chen@...ux.intel.com, valentin.schneider@....com,
        qais.yousef@....com, linux-pm@...r.kernel.org
Cc:     peterz@...radead.org, vincent.guittot@...aro.org, pavel@....cz,
        David.Laight@...LAB.COM, mingo@...hat.com,
        morten.rasmussen@....com, pjt@...gle.com, dietmar.eggemann@....com,
        tj@...nel.org, rafael.j.wysocki@...el.com,
        daniel.lezcano@...aro.org, dhaval.giani@...cle.com,
        qperret@...rret.net, ggherdovich@...e.cz, viresh.kumar@...aro.org,
        Doug Smythies <dsmythies@...us.net>
Subject: Re: [Discussion v2] Usecases for the per-task latency-nice attribute



On 9/30/19 4:13 PM, Parth Shah wrote:
> Hello everyone,
> 
> This is the v2 of the discussion started for introducing per-task
> latency-nice attribute for providing scheduler hints.
> 
> v1: https://lkml.org/lkml/2019/9/18/555
> 
> In brief, we face two challenges with the introduction of such attr.
> 
> 1. Name:
> ==============
> ( Should be relevant to all the possible usecases, not confuse end-user and
> reflect the functionality it provides to the scheduler behaviour )
> 
> Curated list of proposed names:
> 
> 1. latency-nice:
>    should have a better understanding based on pre-existing concepts
> 
> - But poses two interpretation ambiguity
>   a) -20 (least nice to latency, i.e. sacrifice latency for throughput)
>      +19 (most nice to latency, i.e. sacrifice throughput for latency)
>   b) -20 (least nice to other task in terms of sacrificing latency, i.e.
> 	  latency-sensitive)
>      +19 (most nice to other tasks in terms of sacrificing latency, i.e.
> 	  latency-forgoing)
> 
> 2. latency-tolerant:
>    decouples a bit its meaning from the niceness thus giving maybe a bit
>    more freedom in its complete definition and perhaps avoid any
>    possible interpretation confusion
> 
> 3. latency-nasty
> 
> 4. latency-sensible

+ 5. temper
     -20 (short temper, angry tasks, i.e., requires least latency)
     +19 (calm tasks, i.e., sacrifice latency for throughput)

> 
> 
> 
> 2. Value(s):
> ==============
> ( Boolean/Ternary, Range of values, profile tagging )
> 
> - Recent discussion plots the range of [-20, 19] to be the most agreed upon.
> 
> 1. Range:
> - [-20, 19]:
>     Which has similarities with the niceness concept and gives a minimal
>     continuous range. This can be on hand for things like scaling the
>     vruntime normalization [3]
> 
> 2. Profile tagging:
> - Can be used just like a flag attribute
>   e.g., Background, foreground, latency-sensible, reduce-idle-search, etc.
> 
> 3. Binary:
> - 0 for: Latency sensitive/sensible/in-tolerant/hungry...
> - 1 for Latency insensitive/insensible/tolerant/nice-to-others/...
> 
>   Ternary:
> -  0: no effect
> - -1: require least latency
> - +1: no restrictions in terms of lower/higher latency
> 
> [...]

I guess the latency-tolerant name seems to be more relevant and the range
[-20,19] will suit all the discussed usecases.
( ( ( tomatoes target here ) ) )

If this seems alright then I am thinking of writing out some patches to
introduce p->latency-tolerant with the use of "sched_setattr" syscall.


Thanks,
Parth