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    SubjectRe: [PATCH 1/1] sched/fair: Fix inaccurate tally of ttwu_move_affine
    On Wed, Aug 10, 2022 at 03:33:13PM -0700, Libo Chen wrote:
    > There are scenarios where non-affine wakeups are incorrectly counted as
    > affine wakeups by schedstats.
    >
    > When wake_affine_idle() returns prev_cpu which doesn't equal to
    > nr_cpumask_bits, it will slip through the check: target == nr_cpumask_bits
    > in wake_affine() and be counted as if target == this_cpu in schedstats.
    >
    > Replace target == nr_cpumask_bits with target != this_cpu to make sure
    > affine wakeups are accurately tallied.
    >
    > Fixes: 806486c377e33 (sched/fair: Do not migrate if the prev_cpu is idle)
    > Suggested-by: Daniel Jordan <daniel.m.jordan@oracle.com>
    > Signed-off-by: Libo Chen <libo.chen@oracle.com>
    > ---
    > kernel/sched/fair.c | 2 +-
    > 1 file changed, 1 insertion(+), 1 deletion(-)
    >
    > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    > index da388657d5ac..b179da4f8105 100644
    > --- a/kernel/sched/fair.c
    > +++ b/kernel/sched/fair.c
    > @@ -6114,7 +6114,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
    > target = wake_affine_weight(sd, p, this_cpu, prev_cpu, sync);
    >
    > schedstat_inc(p->stats.nr_wakeups_affine_attempts);
    > - if (target == nr_cpumask_bits)
    > + if (target != this_cpu)
    > return prev_cpu;


    This seems to be the right thing to do. However,..

    if this_cpu and prev_cpu were in the same LLC and we pick prev_cpu,
    technically is it still not an affine wakeup?


    >
    > schedstat_inc(sd->ttwu_move_affine);
    > --
    > 2.31.1
    >

    --
    Thanks and Regards
    gautham.

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