Re: [PATCH v5 0/4] Introduce per-task latency_nice for scheduler hints

[chris hyser <chris.hyser@oracle.com>]

On 2/28/20 4:07 AM, Parth Shah wrote:
> This is the 5th revision of the patch set to introduce latency_nice 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
> v3: https://lkml.org/lkml/2020/1/16/319
> v4: https://lkml.org/lkml/2020/2/24/216
> 
> Changes in this revision are:
> v4->v5:
> - Added debugging prints in /proc/<pid>/sched for latency_nice ( based on
>    suggestion from Pavan Kondeti )
> - Initialized init_task with latency_nice = 0
> - Collected review tag and added few minor fixes.
> v3->v4:
> - Based on Chris's comment, added security check to refrain non-root user
>    set lower value than the current latency_nice values.
> 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 1-3: Add support for latency_nice attr in the task struct using
>    	     sched_{set,get}attr syscall
> - Patch 4  : Add permission checks for setting the value.
> 
> 
> The patch series can be applied on tip/sched/core at the
> commit a0f03b617c3b ("sched/numa: Stop an exhastive search if a reasonable swap candidate or idle CPU is found")
> 
> 
> 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 (4):
>    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
>    sched/core: Add permission checks for setting the latency_nice value
> 
>   include/linux/sched.h            |  1 +
>   include/uapi/linux/sched.h       |  4 +++-
>   include/uapi/linux/sched/types.h | 19 +++++++++++++++++++
>   init/init_task.c                 |  1 +
>   kernel/sched/core.c              | 26 ++++++++++++++++++++++++++
>   kernel/sched/debug.c             |  1 +
>   kernel/sched/sched.h             | 18 ++++++++++++++++++
>   tools/include/uapi/linux/sched.h |  4 +++-
>   8 files changed, 72 insertions(+), 2 deletions(-)

I've now had a chance to test and play with this a fair amount. It looks good, meets the need. Thanks Parth.

-chrish