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    SubjectRe: [PATCH] sched/fair: check for idle core
    On Wed, 21 Oct 2020 at 17:08, Mel Gorman <mgorman@suse.de> wrote:
    >
    > On Wed, Oct 21, 2020 at 03:24:48PM +0200, Julia Lawall wrote:
    > > > I worry it's overkill because prev is always used if it is idle even
    > > > if it is on a node remote to the waker. It cuts off the option of a
    > > > wakee moving to a CPU local to the waker which is not equivalent to the
    > > > original behaviour.
    > >
    > > But it is equal to the original behavior in the idle prev case if you go
    > > back to the runnable load average days...
    > >
    >
    > It is similar but it misses the sync treatment and sd->imbalance_pct part of
    > wake_affine_weight which has unpredictable consequences. The data
    > available is only on the fully utilised case.

    In fact It's the same because runnable_load_avg was null when cpu is idle, so
    if prev_cpu was idle, we were selecting prev_idle

    >
    > > The problem seems impossible to solve, because there is no way to know by
    > > looking only at prev and this whether the thread would prefer to stay
    > > where it was or go to the waker.
    > >
    >
    > Yes, this is definitely true. Looking at prev_cpu and this_cpu is a
    > crude approximation and the path is heavily limited in terms of how
    > clever it can be.
    >
    > --
    > Mel Gorman
    > SUSE Labs

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