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    SubjectRe: [PATCH RFC] Priority boosting for preemptible RCU
    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:
    > > On Thu, Aug 23, 2007 at 01:54:56AM -0700, Paul E. McKenney wrote:
    > > > On Thu, Aug 23, 2007 at 09:56:39AM +0530, Gautham R Shenoy wrote:
    > > > >
    > > > > 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?

    >
    > > > 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!"
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