Re: [PATCH RFC] v1 expedited "big hammer" RCU grace periods

[Ingo Molnar <mingo@elte.hu>]

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> First cut of "big hammer" expedited RCU grace periods, but only 
> for rcu_bh.  This creates another softirq vector, so that entering 
> this softirq vector will have forced an rcu_bh quiescent state (as 
> noted by Dave Miller).  Use smp_call_function() to invoke 
> raise_softirq() on all CPUs in order to cause this to happen.  
> Track the CPUs that have passed through a quiescent state (or gone 
> offline) with a cpumask.
> 
> Does nothing to expedite callbacks already registered with 
> call_rcu_bh(), but there is no need to.
> 
> Shortcomings:
> 
> o	Untested, probably does not compile, not for inclusion.
> 
> o	Does not handle rcu, only rcu_bh.
> 
> Thoughts?

I'm wondering, why not just do a two-liner, along the lines of:

	for_each_online_cpu(cpu)
		smp_send_reschedule(cpu);

That should trigger a quiescent state on all online cpus. It wont 
perturb the scheduler state (which is reschedule-IPI invariant).
(And this is a big-hammer approach anyway so even if it did we 
wouldnt care.)

Am i missing something embarrasingly obvious perhaps?

	Ingo