On Thu, Aug 23, 2007 at 06:15:01AM -0700, Paul E. McKenney wrote:

On Thu, Aug 23, 2007 at 03:44:44PM +0530, Gautham R Shenoy wrote:

quoted

On Thu, Aug 23, 2007 at 01:54:56AM -0700, Paul E. McKenney wrote:

quoted

On Thu, Aug 23, 2007 at 09:56:39AM +0530, Gautham R Shenoy wrote:

quoted

I feel we should still be able to use for_each_online_cpu(cpu) instead
of for_each_possible_cpu. Again, there's a good chance that I might
be mistaken!

How about the following ?

	preempt_disable(); /* We Dont want cpus going down here */
	for_each_online_cpu(cpu) 
		for (i = 0; i < RCU_BOOST_ELEMENTS; i++) {
			rbdp = per_cpu(rcu_boost_dat, cpu);
			sum.rbs_blocked += rbdp[i].rbs_blocked;
			sum.rbs_boost_attempt += rbdp[i].rbs_boost_attempt;
			sum.rbs_boost += rbdp[i].rbs_boost;
			sum.rbs_unlock += rbdp[i].rbs_unlock;
			sum.rbs_unboosted += rbdp[i].rbs_unboosted;
		}
	preempt_enable(); 


	static int rcu_boost_cpu_callback(struct notifier_bloack *nb, 
					unsigned long action, void *hcpu) 
	{
		int this_cpu, cpu;
		rcu_boost_data *rbdp, *this_rbdp;

		switch (action) {
		case CPU_DEAD:
			this_cpu = get_cpu();
			cpu = (long)hcpu;
			this_cpu = smp_processor_id();
			rbdp = per_cpu(rcu_boost_dat, cpu);
			this_rbdp = per_cpu(rcu_boost_dat, cpu);
			/* 
			 *  Transfer all of rbdp's statistics to
			 *  this_rbdp here.
			 */	
			 put_cpu();
	
			return NOTIFY_OK;
		}
	}


Won't this work in this case?

Hello, Gautham,

We could do something similar.  If there was a global rcu_boost_data
variable that held the sums of the fields of the rcu_boost_data
structures for all offline CPUs, and if we used a new lock to protect
that global rcu_boost data variable (both when reading and when
CPU hotplugging), then we could indeed scan only the online CPUs'
rcu_boost_data elements.

We would also have to maintain a cpumask_t for this purpose, and
we would need to add a CPU's contribution when it went offline and
subtract it when that CPU came back online.

The additional cpumask_t beats me though! Doesn't the cpu_online_map
suffice here? 
The addition and subtraction of a hotplugged cpu's
contribution from the global rcu_boost_data could be done while
handling the CPU_ONLINE and CPU_DEAD (or CPU_UP_PREPARE
and CPU_DOWN_PREPARE, whichever suits better), in the cpu hotplug
callback. 

Am I missing something ?

Don't we need to synchronize the manipulation of the hotplugged CPU's
contribution and the manipulation of cpu_online_map?  Otherwise, if
stats are called for just before (or just after, depending on the
ordering of hotplug operations) the invocation will get the wrong
statistics.

Oh, yes we need to synchronize that :-)

Can't we use lock_cpu_hotplug/unlock_cpu_hotplug (or it's variant when 
it is available) around any access to cpu_online_map ? With that, it's
guaranteed that no cpu-hotplug operation will be permitted while you're
iterating over the cpu_online_map, and hence you have a  
consistent view of global rcu_boost_data.

Even if we use another cpumask_t, whenever a cpu goes down or comes up,
that will be reflected in this map, no? So what's the additional
advantage of using it?

quoted

quoted

The lock should not be a problem even on very large systems because
of the low frequency of statistics printing -- and of hotplug operations,
for that matter.

The lock is not a problem, so long as somebody else doesn't call
the function taking the lock from their cpu-hotplug callback path :-)
Though I don't see it happening here.

There are some ways to decrease its utilization if it should become
a problem in any case.

Cool!

						Thanx, Paul

Thanks and Regards
gautham.
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"