Hi Peter,

  What should I do next? Thanks.

Cheng

on 09/14/2016 10:01 AM, Cheng Chao wrote:
> In case @cpu == smp_proccessor_id(), we can avoid a sleep+wakeup
> by doing a preemption.
> 
> the caller such as sched_exec can benefit from this change.
> 
> Signed-off-by: Cheng Chao <cs.os.ker...@gmail.com>
> Cc: Oleg Nesterov <o...@redhat.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> ---
>  kernel/sched/core.c   | 8 ++++++--
>  kernel/stop_machine.c | 5 +++++
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a0086a5..283b662 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1063,8 +1063,12 @@ static int migration_cpu_stop(void *data)
>        * holding rq->lock, if p->on_rq == 0 it cannot get enqueued because
>        * we're holding p->pi_lock.
>        */
> -     if (task_rq(p) == rq && task_on_rq_queued(p))
> -             rq = __migrate_task(rq, p, arg->dest_cpu);
> +     if (task_rq(p) == rq) {
> +             if (task_on_rq_queued(p))
> +                     rq = __migrate_task(rq, p, arg->dest_cpu);
> +             else
> +                     p->wake_cpu = arg->dest_cpu;
> +     }
>       raw_spin_unlock(&rq->lock);
>       raw_spin_unlock(&p->pi_lock);
>  
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index 4a1ca5f..1a24890 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -126,6 +126,11 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, 
> void *arg)
>       cpu_stop_init_done(&done, 1);
>       if (!cpu_stop_queue_work(cpu, &work))
>               return -ENOENT;
> +     /*
> +      * In case @cpu == smp_proccessor_id() we can avoid a sleep+wakeup
> +      * by doing a preemption.
> +      */
> +     cond_resched();
>       wait_for_completion(&done.completion);
>       return done.ret;
>  }
> 

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