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 <[EMAIL PROTECTED]> --- 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/