linux-kernel - Re: [PATCH RFC] Priority boosting for preemptible RCU

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Message-ID: <20070823085456.GA18627@linux.vnet.ibm.com>
Date:	Thu, 23 Aug 2007 01:54:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Gautham R Shenoy <ego@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	mingo@...e.hu, akpm@...ux-foundation.org, dipankar@...ibm.com,
	josht@...ux.vnet.ibm.com, tytso@...ibm.com, dvhltc@...ibm.com,
	tglx@...utronix.de
Subject: Re: [PATCH RFC] Priority boosting for preemptible RCU

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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