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    SubjectRe: [RFC PATCH 9/9] sched: rotate the cpu search window for better spread
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    On 8/30/19 11:19 PM, subhra mazumdar wrote:
    > Rotate the cpu search window for better spread of threads. This will ensure
    > an idle cpu will quickly be found if one exists.
    >
    > Signed-off-by: subhra mazumdar <subhra.mazumdar@oracle.com>
    > ---
    > kernel/sched/fair.c | 10 ++++++++--
    > 1 file changed, 8 insertions(+), 2 deletions(-)
    >
    > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    > index 94dd4a32..7419b47 100644
    > --- a/kernel/sched/fair.c
    > +++ b/kernel/sched/fair.c
    > @@ -6188,7 +6188,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
    > u64 avg_cost, avg_idle;
    > u64 time, cost;
    > s64 delta;
    > - int cpu, floor, nr = INT_MAX;
    > + int cpu, floor, target_tmp, nr = INT_MAX;
    >
    > this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
    > if (!this_sd)
    > @@ -6213,9 +6213,15 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
    > nr = floor;
    > }
    >
    > + if (per_cpu(next_cpu, target) != -1)
    > + target_tmp = per_cpu(next_cpu, target);
    > + else
    > + target_tmp = target;
    > +
    > time = local_clock();
    >
    > - for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
    > + for_each_cpu_wrap(cpu, sched_domain_span(sd), target_tmp) {
    > + per_cpu(next_cpu, target) = cpu;

    Is it possible that two simultaneous select_idle_cpu call have the same target value?

    > if (!--nr)
    > return -1;
    > if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
    >

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