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

Peter Zijlstra
2011-05-30T16:47:08Z

On Mon, 2011-05-30 at 09:23 -0700, Paul E. McKenney wrote:

> > @@ -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:
>

well, no that would get us back to waking a task affine to an offline
cpu :-)

> > @@ -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?

Ah, not so, see the initial one is conditional on cpu_online() and will
fail for the CPU_UP_PREPARE case, this new function will be ran from
CPU_ONLINE to then issue the first wakeup.

The distinction comes from the initialize while cpus are already running
vs hotplug.