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    SubjectRe: [PATCH -tip 09/32] sched/fair: Snapshot the min_vruntime of CPUs on force idle
    On Mon, Nov 23, 2020 at 07:31:31AM -0500, Vineeth Pillai wrote:
    > Hi Balbir,
    >
    > On 11/22/20 6:44 AM, Balbir Singh wrote:
    > >
    > > This seems cumbersome, is there no way to track the min_vruntime via
    > > rq->core->min_vruntime?
    > Do you mean to have a core wide min_vruntime? We had a
    > similar approach from v3 to v7 and it had major issues which
    > broke the assumptions of cfs. There were some lengthy
    > discussions and Peter explained in-depth about the issues:
    >
    > https://lwn.net/ml/linux-kernel/20200506143506.GH5298@hirez.programming.kicks-ass.net/
    > https://lwn.net/ml/linux-kernel/20200515103844.GG2978@hirez.programming.kicks-ass.net/
    >

    One of the equations in the link is

    ">From which immediately follows that:

    T_k + T_l
    S_k+l = --------- (13)
    W_k + W_l

    On which we can define a combined lag:

    lag_k+l(i) := S_k+l - s_i (14)

    And that gives us the tools to compare tasks across a combined runqueue.
    "

    S_k+l reads like rq->core->vruntime, but it sounds like the equivalent
    of rq->core->vruntime is updated when we enter forced idle as opposed to
    all the time.

    Balbir

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