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

Paul E. McKenney
2011-05-30T16:41:45Z

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
> >