Re: [PATCH v5 5/7] sched/fair: Add sched group latency support

[Qais Yousef <qais.yousef@arm.com>]

On 09/25/22 16:39, Vincent Guittot wrote:
> Task can set its latency priority with sched_setattr(), which is then used
> to set the latency offset of its sched_entity, but sched group entities
> still have the default latency offset value.
> 
> Add a latency.nice field in cpu cgroup controller to set the latency
> priority of the group similarly to sched_setattr(). The latency priority
> is then used to set the offset of the sched_entities of the group.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  Documentation/admin-guide/cgroup-v2.rst |  8 ++++
>  kernel/sched/core.c                     | 53 +++++++++++++++++++++++++
>  kernel/sched/fair.c                     | 33 +++++++++++++++
>  kernel/sched/sched.h                    |  4 ++
>  4 files changed, 98 insertions(+)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index be4a77baf784..d8ae7e411f9c 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1095,6 +1095,14 @@ All time durations are in microseconds.
>          values similar to the sched_setattr(2). This maximum utilization
>          value is used to clamp the task specific maximum utilization clamp.
>  
> +  cpu.latency.nice
> +	A read-write single value file which exists on non-root
> +	cgroups.  The default is "0".
> +
> +	The nice value is in the range [-20, 19].
> +
> +	This interface file allows reading and setting latency using the
> +	same values used by sched_setattr(2).

I still don't understand how tasks will inherit the latency_nice value from
cgroups they're attached to.

For example, in EAS path we operate at task level only. If the task's
p->latency_nice = 0, but it belongs to a task group tg->latency_nice = -19;
what should the task's latency_nice be in this case? If it's in a hierarchy,
how would the effective value be calculated?


Thanks

--
Qais Yousef