Re: [PATCH 1/3] sched: Add new API wake_up_if_idle() to wake up the idle cpu

Andy Lutomirski
2014-08-21T21:22:14Z

On Aug 18, 2014 3:52 AM, "Chuansheng Liu" <chuansheng.liu@intel.com> wrote:
>
> Implementing one new API wake_up_if_idle(), which is used to
> wake up the idle CPU.
>
> Suggested-by: Andy Lutomirski <luto@amacapital.net>
> Signed-off-by: Chuansheng Liu <chuansheng.liu@intel.com>
> ---
> include/linux/sched.h | 1 +
> kernel/sched/core.c | 16 ++++++++++++++++
> 2 files changed, 17 insertions(+)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 857ba40..3f89ac1 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1024,6 +1024,7 @@ struct sched_domain_topology_level {
> extern struct sched_domain_topology_level *sched_domain_topology;
>
> extern void set_sched_topology(struct sched_domain_topology_level *tl);
> +extern void wake_up_if_idle(int cpu);
>
> #ifdef CONFIG_SCHED_DEBUG
> # define SD_INIT_NAME(type) .name = #type
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1211575..adf104f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1620,6 +1620,22 @@ static void ttwu_queue_remote(struct task_struct *p, int cpu)
> }
> }
>
> +void wake_up_if_idle(int cpu)
> +{
> + struct rq *rq = cpu_rq(cpu);
> + unsigned long flags;
> +
> + if (set_nr_if_polling(rq->idle)) {
> + trace_sched_wake_idle_without_ipi(cpu);
> + } else {
> +

FWIW, adding:

if (rq->curr != rq->idle)
return;

Right here could improve performance on large, mostly non-idle
systems. It would skip the spinlock in most cases.

I don't know whether this code is relevant on systems like that, though.

raw_spin_lock_irqsave(&rq->lock, flags);
> + if (rq->curr == rq->idle)
> + smp_send_reschedule(cpu);
> + /* Else cpu is not in idle, do nothing here */
> + raw_spin_unlock_irqrestore(&rq->lock, flags);
> + }
> +

--Andy