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    SubjectRe: [patch 1/2] sched: use resched IPI to kick off the nohz idle balance
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    Something like so in fact,.. except for the MB bits..

    ---
    Index: linux-2.6/kernel/sched.c
    ===================================================================
    --- linux-2.6.orig/kernel/sched.c
    +++ linux-2.6/kernel/sched.c
    @@ -1404,6 +1404,18 @@ void wake_up_idle_cpu(int cpu)
    smp_send_reschedule(cpu);
    }

    +static inline bool got_nohz_kick(void)
    +{
    + return !!this_rq()->nohz_balance_kick;
    +}
    +
    +#else /* CONFIG_NO_HZ */
    +
    +static inline bool got_nohz_kick(void)
    +{
    + return false;
    +}
    +
    #endif /* CONFIG_NO_HZ */

    static u64 sched_avg_period(void)
    @@ -2717,7 +2729,7 @@ static void sched_ttwu_pending(void)

    void scheduler_ipi(void)
    {
    - if (llist_empty(&this_rq()->wake_list))
    + if (llist_empty(&this_rq()->wake_list) && got_nohz_kick())
    return;

    /*
    @@ -2734,7 +2746,14 @@ void scheduler_ipi(void)
    * somewhat pessimize the simple resched case.
    */
    irq_enter();
    - sched_ttwu_pending();
    + if (list)
    + sched_ttwu_do_pending(list);
    +
    + /*
    + * Check if someone kicked us for doing the nohz idle load balance.
    + */
    + if (unlikely(got_nohz_kick() && idle_cpu(smp_processor_id()) && !need_resched()))
    + raise_softirq_irqoff(SCHED_SOFTIRQ);
    irq_exit();
    }

    @@ -8288,7 +8307,6 @@ void __init sched_init(void)
    rq_attach_root(rq, &def_root_domain);
    #ifdef CONFIG_NO_HZ
    rq->nohz_balance_kick = 0;
    - init_sched_softirq_csd(&per_cpu(remote_sched_softirq_cb, i));
    #endif
    #endif
    init_rq_hrtick(rq);
    Index: linux-2.6/kernel/sched_fair.c
    ===================================================================
    --- linux-2.6.orig/kernel/sched_fair.c
    +++ linux-2.6/kernel/sched_fair.c
    @@ -4269,22 +4269,6 @@ static int active_load_balance_cpu_stop(
    }

    #ifdef CONFIG_NO_HZ
    -
    -static DEFINE_PER_CPU(struct call_single_data, remote_sched_softirq_cb);
    -
    -static void trigger_sched_softirq(void *data)
    -{
    - raise_softirq_irqoff(SCHED_SOFTIRQ);
    -}
    -
    -static inline void init_sched_softirq_csd(struct call_single_data *csd)
    -{
    - csd->func = trigger_sched_softirq;
    - csd->info = NULL;
    - csd->flags = 0;
    - csd->priv = 0;
    -}
    -
    /*
    * idle load balancing details
    * - One of the idle CPUs nominates itself 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.
    + */
    + kick_process(idle_task(ilb_cpu));
    }
    return;
    }


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