Re: [PATCH 11/41] nohz/cpuset: Don't turn off the tick if rcu needsit

[Frederic Weisbecker]

On Wed, May 23, 2012 at 09:27:39AM -0700, Paul E. McKenney wrote:
> On Wed, May 23, 2012 at 06:06:33PM +0200, Frederic Weisbecker wrote:
> > On Wed, May 23, 2012 at 08:15:42AM -0700, Paul E. McKenney wrote:
> > > On Wed, May 23, 2012 at 03:52:09PM +0200, Frederic Weisbecker wrote:
> > > > > > +#ifdef CONFIG_CPUSETS_NO_HZ
> > > > > > +static bool can_stop_adaptive_tick(void)
> > > > > > +{
> > > > > > +	if (!sched_can_stop_tick())
> > > > > > +		return false;
> > > > > > +
> > > > > > +	/* Is there a grace period to complete ? */
> > > > > > +	if (rcu_pending(smp_processor_id()))
> > > > > 
> > > > > You lost me on this one.  Why can't this be rcu_needs_cpu()?
> > > > 
> > > > We already have an rcu_needs_cpu() check in tick_nohz_stop_sched_tick()
> > > > that prevents the tick to shut down if the CPU has local callbacks to handle.
> > > > 
> > > > The rcu_pending() check is there in case some other CPU is waiting for the
> > > > current one to help completing a grace period, by reporting a quiescent state
> > > > for example. This happens because we may stop the tick in the kernel, not only
> > > > userspace. And if we are in the kernel, we still need to be part of the global
> > > > state machine.
> > > 
> > > Ah!  But RCU will notice that the CPU is in dyntick-idle mode, and will
> > > therefore take any needed quiescent-state action on that CPU's behalf.
> > > So there should be no need to call rcu_pending() anywhere outside of the
> > > RCU core code.
> > 
> > No. If the tick is stopped and we are in the kernel, we may be using RCU
> > anytime, so we need to be part of the RCU core.
> 
> OK, so the only problem is if we spend a long time CPU-bound in the kernel,
> where "long" is milliseconds or tens of milliseconds.  In that case, the
> RCU core will notice that the CPU has not responded but is not idle, for
> example, in rcu_implicit_dynticks_qs().  It can take action at this point
> to get the offending CPU to pay attention to RCU.
> 
> Does this make sense, or am I still missing something?

Yeah that's exactly the purpose of the rcu_pending() check before shutting down
the tick and the IPI to wake it up.
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