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    SubjectRe: [RFC 6/6] sched/fair: Add sched group latency support
    Hi Tejun,

    On Mon, 21 Mar 2022 at 18:24, Tejun Heo <tj@kernel.org> 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

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