Re: Very high CPU load when idle with 3.0-rc1

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, May 30, 2011 at 09:23:54AM -0700, Paul E. McKenney wrote:
> On Mon, May 30, 2011 at 01:34:51PM +0200, Peter Zijlstra wrote:
> > On Mon, 2011-05-30 at 07:59 +0200, Damien Wyart wrote:
> > > Hi,
> > > 
> > > Testing 3.0-rc1 on a core i7 (4 cores + HT), I get a load average of 9.0
> > > when idle. No process is shown running or in "D state" in htop. The box
> > > is behaving normal, no impression of lag or slowness.
> > > 
> > > Not sure what other info to include, I guess this should be quite easy
> > > to reproduce.
> > 
> > 
> > ---
> > Subject: rcu: Cure load woes
> > 
> > Commit cc3ce5176d83 (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: Damien Wyart <damien.wyart@free.fr>
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> 
> Very cool!  I do have a few questions below, but am queuing and testing
> this in the meantime.
> 
> > ---
> >  kernel/rcutree.c        |   54 ++++++++++++++++++++++++++++++++++++++++-------
> >  kernel/rcutree_plugin.h |   11 ++++++++-
> >  2 files changed, 56 insertions(+), 9 deletions(-)
> > 
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 77a7671..89419ff 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -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);
> > +		}
> 
> Would it be OK to simplify the code a bit by doing this initial wakeup
> in rcu_spawn_one_cpu_kthread() itself?  My thought would be to rearrange
> rcu_spawn_one_cpu_kthread() as follows:
> 
> static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu)
> {
> 	struct sched_param sp;
> 	struct task_struct *t;
> 
> 	if (!rcu_kthreads_spawnable ||
> 	    per_cpu(rcu_cpu_kthread_task, cpu) != NULL)
> 		return 0;
> 	t = kthread_create(rcu_cpu_kthread, (void *)(long)cpu, "rcuc%d", 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);
> 	sp.sched_priority = RCU_KTHREAD_PRIO;
> 	sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
> 	wake_up_process(t);
> 	per_cpu(rcu_cpu_kthread_task, cpu) = t;
> 	return 0;
> }
> 
> >  	}
> >  	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);
> 
> Ditto here -- can this wake_up_process() be pushed into
> rcu_spawn_one_node_kthread()?
> 
> >  	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);
> > +		}
> 
> Analogous question here for rcu_wake_one_boost_kthread being eliminated
> in favor of doing the wake_up_process() in rcu_spawn_one_node_kthread().
> 
> >  	}
> >  	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)
> 
> Indeed, this naming is much better than mine.  ;-)
> 
> >  {
> >  	struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
> >  	struct rcu_node *rnp = rdp->mynode;
> > @@ -2209,6 +2221,31 @@ 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);
> 
> Interesting...  So we are really awakening them twice, once at creation
> time to get them to sleep interruptibly, and a second time when the CPU
> comes online.
> 
> What does this second set of wake_up_process() calls do?
> 
> > +}
> > +
> > +/*
> >   * Handle CPU online/offline notification events.
> >   */
> >  static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
> > @@ -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);
> > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > index a767b7d..2910de7 100644
> > --- a/kernel/rcutree_plugin.h
> > +++ b/kernel/rcutree_plugin.h
> > @@ -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_thread_task);

And this needs to be:

	wake_up_process(rnp->boost_kthread_task);

I fixed this in my tree, continuing testing.

							Thanx, Paul

> > +}
> > +
> >  #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
> >