Re: [RFC 6/6] sched/fair: Add sched group latency support

[Vincent Guittot]

Hi Tejun,

On Mon, 21 Mar 2022 at 18:24, Tejun Heo <tj@xxxxxxxxxx> wrote:
>
> Hello,
>
> On Fri, Mar 11, 2022 at 05:14:06PM +0100, Vincent Guittot wrote:
> > Tasks can set its latency priority which is then used to decide to preempt
> > the current running entity of the cfs but sched group entities still have
> > the default latency priority.
> >
> > Add a latency field in task group to set the latency priority of the group
> > which will be used against other entity in the parent cfs.
>
> One thing that bothers me about this interface is that the configuration
> values aren't well defined. We have the same problems with the nice levels
> but at least have them map to well defined weight values, which likely won't
> change in any foreseeable future. The fact that we have the
> not-too-well-defined nice levels as an interface shouldn't be a reason we
> add another one. Provided that this is something scheduler folks want, it'd
> be really great if the interface can be better thought through. What are the
> numbers actually encoding?

latency_nice is quite similar to nice. The nice latency is used as an
index to get a latency weight in the range [-1024:1024].  latency_nice
is in the range [-20:19] and latency_prio shifts it in the range
[0:40] . This index is then used to get the latency weight similar to
how the nice prio is used to get a weight. That being said, the
latency should probably reflect the latency_weight instead of the
latency_prio in order to be aligned with the weight and weight.nice
fields of cgroups.

As described in patch 5 commit message, the weight is then used to
compute a relative offset to check whether the waking task can preempt
the current running task.

Vincent

>
> Thanks.
>
> --
> tejun