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Date:   Mon, 17 Feb 2020 14:27:22 +0530
From:   Parth Shah <parth@...ux.ibm.com>
To:     vincent.guittot@...aro.org, patrick.bellasi@...bug.net,
        valentin.schneider@....com, dhaval.giani@...cle.com,
        dietmar.eggemann@....com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...hat.com, qais.yousef@....com, pavel@....cz,
        qperret@...rret.net, David.Laight@...LAB.COM, pjt@...gle.com,
        tj@...nel.org
Subject: Re: [PATCH v3 0/3] Introduce per-task latency_nice for scheduler
 hints



On 1/16/20 5:32 PM, Parth Shah wrote:
> This is the 3rd revision of the patch set to introduce
> latency_{nice/tolerance} as a per task attribute.
> 
> The previous version can be found at:
> v1: https://lkml.org/lkml/2019/11/25/151
> v2: https://lkml.org/lkml/2019/12/8/10
> 
> Changes in this revision are:
> v2 -> v3:
> - This series changes the longer attribute name to "latency_nice" as per
>   the comment from Dietmar Eggemann https://lkml.org/lkml/2019/12/5/394
> 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
> 
> 
> Introduction:
> ==============
> This patch series introduces a new per-task attribute latency_nice to
> provide the scheduler hints about the latency requirements of the task [1].
> 
> Latency_nice 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_nice = -20"
> should have lower latency requirements than compared to those tasks with
> higher values. Similarly a task with "latency_nice = 19" can have higher
> latency and hence such tasks may not care much about latency.
> 
> The default value is set to 0. The usecases discussed below can use this
> range of [-20, 19] for latency_nice 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_nice during task wakeup
> 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 for 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_nice values.
> 
> 2 TurboSched:
> - Classify the tasks with higher latency_nice 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_nice >= some_threshold (e.g, == 19)
>   and util <= 12.5% can be background tasks.
> 
> 3 Optimize AVX512 based workload:
> - Bias scheduler to not put a task having (latency_nice == -20) on a
>   core occupying AVX512 based workload.
> 
> 
> Series Organization:
> ====================
> - Patch 1: Add new attribute latency_nice to task_struct.
> - Patch 2: Clone parent task's attribute to the child task on fork
> - Patch 3: Add support for sched_{set,get}attr syscall to modify
>   	     latency_nice of the task
> 
> 
> The patch series can be applied on tip/sched/core at the
> commit 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
> 
> 
> References:
> ============
> [1]. Usecases for the per-task latency-nice attribute,
>      https://lkml.org/lkml/2019/9/30/215
> [2]. Task Latency-nice, "Subhra Mazumdar",
>      https://lkml.org/lkml/2019/8/30/829
> [3]. Introduce per-task latency_tolerance for scheduler hints,
>      https://lkml.org/lkml/2019/12/8/10
> 
> 
> Parth Shah (3):
>   sched: Introduce latency-nice as a per-task attribute
>   sched/core: Propagate parent task's latency requirements to the child
>     task
>   sched: Allow sched_{get,set}attr to change latency_nice of the task
> 
>  include/linux/sched.h            |  1 +
>  include/uapi/linux/sched.h       |  4 +++-
>  include/uapi/linux/sched/types.h | 19 +++++++++++++++++++
>  kernel/sched/core.c              | 21 +++++++++++++++++++++
>  kernel/sched/sched.h             | 18 ++++++++++++++++++
>  tools/include/uapi/linux/sched.h |  4 +++-
>  6 files changed, 65 insertions(+), 2 deletions(-)
> 

Its been a long time and few revisions since the beginning of the
discussion around the latency-nice. Hence thought of asking if there is/are
any further work that needs to be done for adding latency-nice attribute or
am I missing any piece in here?


Thanks,
Parth

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