Re: [RFC PATCH 06/15] nohz_task: Keep the tick if rcu needs it

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Dec 20, 2010 at 04:58:20PM +0100, Peter Zijlstra wrote:
> On Mon, 2010-12-20 at 16:24 +0100, Frederic Weisbecker wrote:
> 
> > @@ -1634,7 +1633,7 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
> >   * by the current CPU, returning 1 if so.  This function is part of the
> >   * RCU implementation; it is -not- an exported member of the RCU API.
> >   */
> > -static int rcu_pending(int cpu)
> > +int rcu_pending(int cpu)
> 
> /me wonders about that comment.
> 
> >  {
> >  	return __rcu_pending(&rcu_sched_state, &per_cpu(rcu_sched_data, cpu)) ||
> >  	       __rcu_pending(&rcu_bh_state, &per_cpu(rcu_bh_data, cpu)) ||
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 6dbae46..45bd6e2 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -2470,10 +2470,16 @@ static void nohz_task_cpu_update(void *unused)
> >  int nohz_task_can_stop_tick(void)
> >  {
> >  	struct rq *rq = this_rq();
> > +	int cpu;
> >  
> >  	if (rq->nr_running > 1)
> >  		return 0;
> >  
> > +	cpu = smp_processor_id();
> > +
> > +	if (rcu_pending(cpu) || rcu_needs_cpu(cpu))
> > +		return 0;
> 
> Arguable, rcu_needs_cpu() should imply rcu_pending(), because if there's
> work still to be done, it needs the cpu, hmm?

There are two cases:

1.	This CPU has callbacks.  In this case, rcu_pending() returns 1.

2.	The RCU core needs something from this CPU.  In this case,
	rcu_pending() returns 1.

The trick is that in dyntick-idle mode, if we have #2 but not #1, other
CPUs can (and will) act on the dyntick-idle CPU's behalf.  However, when
there is a task running, that task might do system calls, which can
queue callbacks and can contain RCU read-side critical sections, neither
of which can happen in dyntick-idle mode.

So the one-task-running-on-this-CPU case above does need special
handling.

> >  	return 1;
> >  }
> >  
> 
> This patch also implies you broke stuff with #4 because it would put the
> machine to sleep while RCU still had bits to do, not very nice.

Hmmm...  I need to look at this after getting some sleep.

							Thanx, Paul