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Subject: Re: [PATCH v2 0/3] Introduce per-task latency_tolerance for scheduler
 hints
To:     Parth Shah <parth@linux.ibm.com>, linux-kernel@vger.kernel.org
Cc:     peterz@infradead.org, mingo@redhat.com, vincent.guittot@linaro.org,
        patrick.bellasi@matbug.net, valentin.schneider@arm.com,
        qais.yousef@arm.com, pavel@ucw.cz, qperret@qperret.net,
        David.Laight@ACULAB.COM, pjt@google.com, tj@kernel.org,
        dietmar.eggemann@arm.com, Chris Deon Hyser <chris.hyser@oracle.com>
References: <20191208060410.17814-1-parth@linux.ibm.com>
From:   Dhaval Giani <dhaval.giani@oracle.com>
Message-ID: <5a13d54a-7e16-8d6c-8362-bd5f056004db@oracle.com>
Date:   Wed, 15 Jan 2020 12:33:40 -0800
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On 12/7/19 10:04 PM, Parth Shah wrote:
> This is the 2nd revision of the patch set to introduce latency_toleranc=
e as
> a per task attribute.
>
> The previous version can be found at:
> v1: https://lkml.org/lkml/2019/11/25/151
>
> Changes in this revision are:
> v1 -> v2:
> - Addressed comments from Qais Yousef
> - As per suggestion from Dietmar, moved content from newly created
>   include/linux/sched/latency_tolerance.h to kernel/sched/sched.h
> - Extend sched_setattr() to support latency_tolerance in tools headers =
UAPI
>
>
> This patch series introduces a new per-task attribute latency_tolerance=
 to
> provide the scheduler hints about the latency requirements of the task =
[1].
>
> Latency_tolerance is a ranged attribute of a task with the value rangin=
g
> 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 =3D -20=
"
> should have lower latency than compared to those tasks with higher valu=
es.
> Similarly a task with "latency_tolerance =3D 19" can have higher latenc=
y and
> hence such tasks may not care much about latency.
>
> The default value is set to 0. The usecases discussed below can use thi=
s
> range of [-20, 19] for latency_tolerance for the specific purpose. This=

> patch does not implement 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 wak=
eup
> and load-balancing is yet to be coded for each of those usecases.
>
> As per my view, this defined attribute can be used in following ways fo=
r a
> some of the usecases:
> 1 Reduce search scan time for select_idle_cpu():
> - Reduce search scans for finding idle CPU for a waking task with lower=

>   latency_tolerance values.
>
> 2 TurboSched:
> - Classify the tasks with higher latency_tolerance values as a small
>   background task given that its historic utilization is very low, for
>   which the scheduler can search for more number of cores to do task
>   packing.  A task with a latency_tolerance >=3D some_threshold (e.g, >=
=3D +18)
>   and util <=3D 12.5% can be background tasks.
>
> 3 Optimize AVX512 based workload:
> - Bias scheduler to not put a task having (latency_tolerance =3D=3D -20=
) on a
>   core occupying AVX512 based workload.

Have you been able to adapt any of these use cases to this new interface?=


Does the interface translate well to them?

Do you have any code that you can share?

Dhaval

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