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SubjectRe: [patch 1/2] sched: use resched IPI to kick off the nohz idle balance
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On Thu, 2011-09-29 at 15:30 -0700, Suresh Siddha wrote:

> ---
> kernel/sched.c | 14 +++++++++++---
> kernel/sched_fair.c | 27 +++++++--------------------
> 2 files changed, 18 insertions(+), 23 deletions(-)
>
> Index: linux-2.6-tip/kernel/sched.c
> ===================================================================
> --- linux-2.6-tip.orig/kernel/sched.c
> +++ linux-2.6-tip/kernel/sched.c
> @@ -2733,7 +2733,7 @@ void scheduler_ipi(void)
> struct rq *rq = this_rq();
> struct task_struct *list = xchg(&rq->wake_list, NULL);
>
> - if (!list)
> + if (!list && !idle_cpu(cpu_of(rq)))
> return;

Why not make that !rq->nohz_balance_kick? (wrapped in a helper for !
CONFIG_NO_HZ)

> /*
> @@ -2750,7 +2750,16 @@ void scheduler_ipi(void)
> * somewhat pessimize the simple resched case.
> */
> irq_enter();
> - sched_ttwu_do_pending(list);
> +
> + if (list)
> + sched_ttwu_do_pending(list);
> +
> + /*
> + * Check if someone kicked us for doing the nohz idle load balance.
> + */
> + if (unlikely((rq->idle == current) && rq->nohz_balance_kick &&
> + !need_resched()))
> + raise_softirq_irqoff(SCHED_SOFTIRQ);

And make that: idle_cpu() && rq->nohz_balance_kick && !need_resched()

All wrapped in #ifdef CONFIG_NO_HZ goo?
> tself as idle load_balancer, while
> @@ -4450,11 +4434,14 @@ static void nohz_balancer_kick(int cpu)
> }
>
> if (!cpu_rq(ilb_cpu)->nohz_balance_kick) {
> - struct call_single_data *cp;
> -
> cpu_rq(ilb_cpu)->nohz_balance_kick = 1;
> - cp = &per_cpu(remote_sched_softirq_cb, cpu);
> - __smp_call_function_single(ilb_cpu, cp, 0);
> + /*
> + * Use kick_process instead of resched_cpu.
> + * This way we generate a sched IPI on the target cpu which
> + * is idle. And the softirq performing nohz idle load balance
> + * will be run before returning from the IPI.
> + */

Shouldn't we have a memory barrier of sorts before sending the IPI?

> + kick_process(idle_task(ilb_cpu));
> }
> return;
> }
>
>



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