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Commit d72bce0e authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

rcu: Cure load woes 



Commit cc3ce517 (rcu: Start RCU kthreads in TASK_INTERRUPTIBLE
state) fudges a sleeping task' state, resulting in the scheduler seeing
a TASK_UNINTERRUPTIBLE task going to sleep, but a TASK_INTERRUPTIBLE
task waking up. The result is unbalanced load calculation.

The problem that patch tried to address is that the RCU threads could
stay in UNINTERRUPTIBLE state for quite a while and triggering the hung
task detector due to on-demand wake-ups.

Cure the problem differently by always giving the tasks at least one
wake-up once the CPU is fully up and running, this will kick them out of
the initial UNINTERRUPTIBLE state and into the regular INTERRUPTIBLE
wait state.

[ The alternative would be teaching kthread_create() to start threads as
  INTERRUPTIBLE but that needs a tad more thought. ]

Reported-by: default avatarDamien Wyart <damien.wyart@free.fr>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: default avatarPaul E. McKenney <paul.mckenney@linaro.org>
Link: http://lkml.kernel.org/r/1306755291.1200.2872.camel@twins


Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 55922c9d

kernel/rcutree.c

+46 −8
Original line number Diff line number Diff line
@@ -1648,7 +1648,6 @@ static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu) 
	if (IS_ERR(t))

		return PTR_ERR(t);

	kthread_bind(t, cpu);

	set_task_state(t, TASK_INTERRUPTIBLE);

	per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;

	WARN_ON_ONCE(per_cpu(rcu_cpu_kthread_task, cpu) != NULL);

	per_cpu(rcu_cpu_kthread_task, cpu) = t;
@@ -1756,7 +1755,6 @@ static int __cpuinit rcu_spawn_one_node_kthread(struct rcu_state *rsp, 
		if (IS_ERR(t))

			return PTR_ERR(t);

		raw_spin_lock_irqsave(&rnp->lock, flags);

		set_task_state(t, TASK_INTERRUPTIBLE);

		rnp->node_kthread_task = t;

		raw_spin_unlock_irqrestore(&rnp->lock, flags);

		sp.sched_priority = 99;
@@ -1765,6 +1763,8 @@ static int __cpuinit rcu_spawn_one_node_kthread(struct rcu_state *rsp, 
	return rcu_spawn_one_boost_kthread(rsp, rnp, rnp_index);

}



static void rcu_wake_one_boost_kthread(struct rcu_node *rnp);



/*

 * Spawn all kthreads -- called as soon as the scheduler is running.

 */
@@ -1772,18 +1772,30 @@ static int __init rcu_spawn_kthreads(void) 
{

	int cpu;

	struct rcu_node *rnp;

	struct task_struct *t;



	rcu_kthreads_spawnable = 1;

	for_each_possible_cpu(cpu) {

		per_cpu(rcu_cpu_has_work, cpu) = 0;

		if (cpu_online(cpu))

		if (cpu_online(cpu)) {

			(void)rcu_spawn_one_cpu_kthread(cpu);

			t = per_cpu(rcu_cpu_kthread_task, cpu);

			if (t)

				wake_up_process(t);

		}

	}

	rnp = rcu_get_root(rcu_state);

	(void)rcu_spawn_one_node_kthread(rcu_state, rnp);

	if (rnp->node_kthread_task)

		wake_up_process(rnp->node_kthread_task);

	if (NUM_RCU_NODES > 1) {

		rcu_for_each_leaf_node(rcu_state, rnp)

		rcu_for_each_leaf_node(rcu_state, rnp) {

			(void)rcu_spawn_one_node_kthread(rcu_state, rnp);

			t = rnp->node_kthread_task;

			if (t)

				wake_up_process(t);

			rcu_wake_one_boost_kthread(rnp);

		}

	}

	return 0;

}
@@ -2188,14 +2200,14 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible) 
	raw_spin_unlock_irqrestore(&rsp->onofflock, flags);

}



static void __cpuinit rcu_online_cpu(int cpu)

static void __cpuinit rcu_prepare_cpu(int cpu)

{

	rcu_init_percpu_data(cpu, &rcu_sched_state, 0);

	rcu_init_percpu_data(cpu, &rcu_bh_state, 0);

	rcu_preempt_init_percpu_data(cpu);

}



static void __cpuinit rcu_online_kthreads(int cpu)

static void __cpuinit rcu_prepare_kthreads(int cpu)

{

	struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);

	struct rcu_node *rnp = rdp->mynode;
@@ -2208,6 +2220,31 @@ static void __cpuinit rcu_online_kthreads(int cpu) 
	}

}



/*

 * kthread_create() creates threads in TASK_UNINTERRUPTIBLE state,

 * but the RCU threads are woken on demand, and if demand is low this

 * could be a while triggering the hung task watchdog.

 *

 * In order to avoid this, poke all tasks once the CPU is fully

 * up and running.

 */

static void __cpuinit rcu_online_kthreads(int cpu)

{

	struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);

	struct rcu_node *rnp = rdp->mynode;

	struct task_struct *t;



	t = per_cpu(rcu_cpu_kthread_task, cpu);

	if (t)

		wake_up_process(t);



	t = rnp->node_kthread_task;

	if (t)

		wake_up_process(t);



	rcu_wake_one_boost_kthread(rnp);

}



/*

 * Handle CPU online/offline notification events.

 */
@@ -2221,10 +2258,11 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self, 
	switch (action) {

	case CPU_UP_PREPARE:

	case CPU_UP_PREPARE_FROZEN:

		rcu_online_cpu(cpu);

		rcu_online_kthreads(cpu);

		rcu_prepare_cpu(cpu);

		rcu_prepare_kthreads(cpu);

		break;

	case CPU_ONLINE:

		rcu_online_kthreads(cpu);

	case CPU_DOWN_FAILED:

		rcu_node_kthread_setaffinity(rnp, -1);

		rcu_cpu_kthread_setrt(cpu, 1);

kernel/rcutree_plugin.h

+10 −1
Original line number Diff line number Diff line
@@ -1295,7 +1295,6 @@ static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp, 
	if (IS_ERR(t))

		return PTR_ERR(t);

	raw_spin_lock_irqsave(&rnp->lock, flags);

	set_task_state(t, TASK_INTERRUPTIBLE);

	rnp->boost_kthread_task = t;

	raw_spin_unlock_irqrestore(&rnp->lock, flags);

	sp.sched_priority = RCU_KTHREAD_PRIO;
@@ -1303,6 +1302,12 @@ static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp, 
	return 0;

}



static void __cpuinit rcu_wake_one_boost_kthread(struct rcu_node *rnp)

{

	if (rnp->boost_kthread_task)

		wake_up_process(rnp->boost_kthread_task);

}



#else /* #ifdef CONFIG_RCU_BOOST */



static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
@@ -1326,6 +1331,10 @@ static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp, 
	return 0;

}



static void __cpuinit rcu_wake_one_boost_kthread(struct rcu_node *rnp)

{

}



#endif /* #else #ifdef CONFIG_RCU_BOOST */



#ifndef CONFIG_SMP