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    SubjectRe: [PATCH RFC] Priority boosting for preemptible RCU
    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 ?


    >
    > 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.


    > 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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