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

Paul E. McKenney
2011-05-30T16:24:07Z

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);
> +}
> +
> #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
>