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

Hi Paul, 
On Wed, Aug 22, 2007 at 12:02:54PM -0700, Paul E. McKenney wrote:

quoted

+/*
+ * Print out RCU booster task statistics at the specified interval.
+ */
+static void rcu_boost_dat_stat_print(void)
+{
+	/* Three decimal digits per byte plus spacing per number and line. */
+	char buf[N_RCU_BOOST_STATE * (sizeof(long) * 3 + 2) + 2];
+	int cpu;
+	int event;
+	int i;
+	static time_t lastprint = 0;
+	struct rcu_boost_dat *rbdp;
+	int state;
+	struct rcu_boost_dat sum;
+
+	/* Wait a graceful interval between printk spamming. */
+
+	if (xtime.tv_sec - lastprint <
+	    CONFIG_PREEMPT_RCU_BOOST_STATS_INTERVAL)
+		return;
+
+	/* Sum up the state/event-independent counters. */
+
+	sum.rbs_blocked = 0;
+	sum.rbs_boost_attempt = 0;
+	sum.rbs_boost = 0;
+	sum.rbs_unlock = 0;
+	sum.rbs_unboosted = 0;
+	for_each_possible_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;
+		}

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 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.

						Thanx, Paul