Re: [patch] latency tracer, 2.6.15-rc7

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

On Tue, Jan 03, 2006 at 07:39:42PM +0530, Dipankar Sarma wrote:
> On Sat, Dec 31, 2005 at 12:14:26PM -0800, Paul E. McKenney wrote:
> > So it seems to me that Linus's patch is part of the solution, but
> > needs to also have a global component, perhaps as follows:
> > 
> > 	if (unlikely(rdp->count > 100)) {
> > 		set_need_resched();
> > 		if (unlikely(rdp->count - rdp->last_rs_count > 1000)) {
> > 			int cpu;
> > 
> > 			rdp->last_rs_count = rdp->count;
> > 			spin_lock_bh(&rcu_bh_state.lock);
> > 			for_each_cpu_mask(cpu, rdp->rcu_bh_state.cpumask)
> > 				smp_send_reschedule(cpu);
> > 			spin_unlock_bh(&rcu_bh_state.lock);
> > 		}
> > 	}
> 
> Yes, something like this that covers corner cases and forces
> queiscent state in all cpus, would be ideal.
> 
> > I am sure that I am missing some interaction or another with tickless
> > idle and CPU hotplug covered.
> 
> It would be safe to miss a cpu or two while sending the resched
> interrupt. So, I don't think we need to worry about tickless
> idle and cpu hotplug.

OK, does that also mean that the spin_lock_bh/spin_unlock_bh are also
unnecessary?  ;-)

> > There also needs to be some adjustment in rcu_do_batch(), which will
> > have to somehow get back to a quiescent state periodically.  Dipankar,
> > Vatsa, thoughts?
> 
> My original thought was to make maxbatch dynamic and automatically
> adjust it depending on the situation. I can try that approach.

Makes sense to me!

							Thanx, Paul