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

Paul E. McKenney
2011-05-30T21:30:04Z

On Mon, May 30, 2011 at 06:46:21PM +0200, Peter Zijlstra wrote:
> 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 :-)

My turn to say d'oh, then!

But I should be able to move them back in under "if (cpu_online(cpu))",
right?

> > > @@ -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.

OK, got it.

Thanx, Paul