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Message-Id: <8ed0f40c-eeb4-c487-5420-a8eb185b5cdd@linux.ibm.com>
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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