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:
> > +/*
> > + * 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
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