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    SubjectRe: [PATCH 1/2] sched: Enforce order of leaf CFS runqueues
    On Mon, 2011-07-18 at 16:24 -0700, Paul Turner wrote:

    > Subject: [PATCH] sched: handle on_list ancestor in leaf_add_cfs_rq()
    > From: Paul Turner <>
    > Date: Mon, 18 Jul 2011 16:08:10 -0700
    > Jan H. Schönherr found that in the case of an on_list ancestor we may
    > incorrectly place the child to the right of a great-ancestor on the list.
    > Consider:
    > A
    > / \ Here, t1A results in A->cfs_rq being on_list, however when
    > B t1A we start enqueuing from C this will not be visible. This is
    > / compounded by the fact that on_list expiration may also be out
    > C of order, punching holes in the tree.
    > /
    > t1C
    > Prevent this by making additions to the leaf_cfs_rq_list position independent.
    > This is done by maintaining additions to this list within the
    > enqueue_task_fair() path, which allows us to always enqueue against the
    > current entity's first on_list ancestor.

    The problem I have with this is that it makes the enqueue more
    expensive. We're now optimizing a relatively slow path (load-balance) at
    the cost of the hottest path in the kernel (enqueue/dequeue).

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