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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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