linux-kernel - Re: Usecases for the per-task latency-nice attribute

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Date:   Wed, 18 Sep 2019 16:42:23 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Patrick Bellasi <patrick.bellasi@....com>,
        Parth Shah <parth@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        subhra mazumdar <subhra.mazumdar@...cle.com>,
        tim.c.chen@...ux.intel.com, mingo@...hat.com,
        morten.rasmussen@....com, dietmar.eggemann@....com, pjt@...gle.com,
        vincent.guittot@...aro.org, quentin.perret@....com,
        dhaval.giani@...cle.com, daniel.lezcano@...aro.org, tj@...nel.org,
        rafael.j.wysocki@...el.com, qais.yousef@....com,
        Patrick Bellasi <patrick.bellasi@...bug.net>
Subject: Re: Usecases for the per-task latency-nice attribute

On 18/09/2019 15:18, Patrick Bellasi wrote:
>> 1. Name: What should be the name for such attr for all the possible usecases?
>> =============
>> Latency nice is the proposed name as of now where the lower value indicates
>> that the task doesn't care much for the latency
> 
> If by "lower value" you mean -19 (in the proposed [-20,19] range), then
> I think the meaning should be the opposite.
> 
> A -19 latency-nice task is a task which is not willing to give up
> latency. For those tasks for example we want to reduce the wake-up
> latency at maximum.
> 
> This will keep its semantic aligned to that of process niceness values
> which range from -20 (most favourable to the process) to 19 (least
> favourable to the process).
> 

I don't want to start a bikeshedding session here, but I agree with Parth
on the interpretation of the values.

I've always read niceness values as
-20 (least nice to the system / other processes)
+19 (most nice to the system / other processes)

So following this trend I'd see for latency-nice:
-20 (least nice to latency, i.e. sacrifice latency for throughput)
+19 (most nice to latency, i.e. sacrifice throughput for latency)

However...

>> But there seems to be a bit of confusion on whether we want biasing as well
>> (latency-biased) or something similar, in which case "latency-nice" may
>> confuse the end-user.
> 
> AFAIU PeterZ point was "just" that if we call it "-nice" it has to
> behave as "nice values" to avoid confusions to users. But, if we come up
> with a different naming maybe we will have more freedom.
> 

...just getting rid of the "-nice" would leave us free not to have to
interpret the values as "nice to / not nice to" :)

> Personally, I like both "latency-nice" or "latency-tolerant", where:
> 
>  - latency-nice:
>    should have a better understanding based on pre-existing concepts
> 
>  - 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 like the one I commented above.
> 
> Fun fact: there was also the latency-nasty proposal from PaulMK :)
> 

[...]

> 
> $> Wakeup path tunings
> ==========================
> 
> Some additional possible use-cases was already discussed in [3]:
> 
>  - dynamically tune the policy of a task among SCHED_{OTHER,BATCH,IDLE}
>    depending on crossing certain pre-configured threshold of latency
>    niceness.
>   
>  - dynamically bias the vruntime updates we do in place_entity()
>    depending on the actual latency niceness of a task.
>   
>    PeterZ thinks this is dangerous but that we can "(carefully) fumble a
>    bit there."
>   
>  - bias the decisions we take in check_preempt_tick() still depending
>    on a relative comparison of the current and wakeup task latency
>    niceness values.

Aren't we missing the point about tweaking the sched domain scans (which
AFAIR was the original point for latency-nice)?

Something like default value is current behaviour and
- Being less latency-sensitive means increasing the scans (e.g. trending
  towards only going through the slow wakeup-path at the extreme setting)
- Being more latency-sensitive means reducing the scans (e.g. trending
  towards a fraction of the domain scanned in the fast-path at the extreme
  setting).

> 

$> Load balance tuning
======================

Already mentioned these in [4]:

- Increase (reduce) nr_balance_failed threshold when trying to active
  balance a latency-sensitive (non-latency-sensitive) task.

- Increase (decrease) sched_migration_cost factor in task_hot() for
  latency-sensitive (non-latency-sensitive) tasks.

>> References:
>> ===========
>> [1]. https://lkml.org/lkml/2019/8/30/829
>> [2]. https://lkml.org/lkml/2019/7/25/296
> 
>   [3]. Message-ID: <20190905114709.GM2349@...ez.programming.kicks-ass.net>
>        https://lore.kernel.org/lkml/20190905114709.GM2349@hirez.programming.kicks-ass.net/
> 

[4]: https://lkml.kernel.org/r/3d3306e4-3a78-5322-df69-7665cf01cc43@arm.com

> 
> Best,
> Patrick
>