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Subject: Re: Very high CPU load when idle with 3.0-rc1
From: Peter Zijlstra <peterz@infradead.org>
To: paulmck@linux.vnet.ibm.com
Cc: Damien Wyart <damien.wyart@free.fr>, Ingo Molnar <mingo@elte.hu>,
        Mike Galbraith <efault@gmx.de>, linux-kernel@vger.kernel.org
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Date: Mon, 30 May 2011 18:46:21 +0200
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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.