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    Subject[patch 1/2] sched: fix idle load balancing in softirqd context
    Periodic load balancing in recent kernels happen in the softirq.
    In certain -rt configurations, these softirqs are handled in softirqd
    context. And hence the check for idle processor was always returning
    busy (as nr_running > 1).

    This patch captures the idle information at the tick and passes this info
    to softirq context through an element 'idle_at_tick' in rq.

    Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
    ---

    diff -pNru linux-2.6.20.x86_64/kernel/sched.c linux/kernel/sched.c
    --- linux-2.6.20.x86_64/kernel/sched.c 2007-02-10 03:41:27.000000000 -0800
    +++ linux/kernel/sched.c 2007-02-16 16:22:44.000000000 -0800
    @@ -240,6 +240,7 @@ struct rq {
    unsigned long raw_weighted_load;
    #ifdef CONFIG_SMP
    unsigned long cpu_load[3];
    + unsigned char idle_at_tick;
    #endif
    unsigned long long nr_switches;

    @@ -3350,12 +3351,7 @@ static void run_rebalance_domains(struct
    struct rq *this_rq = cpu_rq(this_cpu);
    unsigned long interval;
    struct sched_domain *sd;
    - /*
    - * We are idle if there are no processes running. This
    - * is valid even if we are the idle process (SMT).
    - */
    - enum idle_type idle = !this_rq->nr_running ?
    - SCHED_IDLE : NOT_IDLE;
    + enum idle_type idle = this_rq->idle_at_tick ? SCHED_IDLE : NOT_IDLE;
    /* Earliest time when we have to call run_rebalance_domains again */
    unsigned long next_balance = jiffies + 60*HZ;

    @@ -3651,20 +3647,21 @@ void scheduler_tick(void)
    {
    unsigned long long now = sched_clock();
    struct task_struct *p = current;
    - int cpu = smp_processor_id();
    + int cpu = smp_processor_id(), idle_at_tick = idle_cpu(cpu);
    struct rq *rq = cpu_rq(cpu);

    BUG_ON(!irqs_disabled());

    update_cpu_clock(p, rq, now);

    - if (p == rq->idle)
    + if (idle_at_tick)
    /* Task on the idle queue */
    wake_priority_sleeper(rq);
    else
    task_running_tick(rq, p);
    #ifdef CONFIG_SMP
    update_load(rq);
    + rq->idle_at_tick = idle_at_tick;
    if (time_after_eq(jiffies, rq->next_balance))
    raise_softirq(SCHED_SOFTIRQ);
    #endif
    -
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