rcu: Improve boost selection when moving tasks to root rcu_node

The rcu_preempt_offline_tasks() moves all tasks queued on a given leaf
rcu_node structure to the root rcu_node, which is done when the last CPU
corresponding the the leaf rcu_node structure goes offline.  Now that
RCU-preempt's synchronize_rcu_expedited() implementation blocks CPU-hotplug
operations during the initialization of each rcu_node structure's
->boost_tasks pointer, rcu_preempt_offline_tasks() can do a better job
of setting the root rcu_node's ->boost_tasks pointer.

The key point is that rcu_preempt_offline_tasks() runs as part of the
CPU-hotplug process, so that a concurrent synchronize_rcu_expedited() is
guaranteed to either have not started on the one hand (in which case there
is no boosting on behalf of the expedited grace period) to be completely
initialized on the other (in which case, in absence of other priority
boosting, all ->boost_tasks pointers will be initialized).  Therefore,
if rcu_preempt_offline_tasks() finds that the ->boost_tasks pointer is
equal to the ->exp_tasks pointer, it can be sure that it is correcty
placed.

The case where there was boosting ongoing at the time that the
synchronize_rcu_expedited() function started, different nodes might
start boosting the tasks blocking the expedited grace period at different
times.  In this mixed case, the root node will either be boosting tasks
for the expedited grace period already, or it will start as soon as it
gets done boosting for the normal grace period -- but in this latter
case, the root node's tasks needed to be boosted in any case.

This commit therefore adds a check of the ->boost_tasks pointer against
the ->exp_tasks pointer to the list that prevents updating ->boost_tasks.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 151d519a1b6b25ed8ba1367371ebd8ca32213174..b47bf53e1fff9beff40330c15a3645e0cc00f972 100644 (file)
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -365,8 +365,11 @@ void rcu_read_unlock_special(struct task_struct *t)
                if (&t->rcu_node_entry == rnp->exp_tasks)
                        rnp->exp_tasks = np;
 #ifdef CONFIG_RCU_BOOST
-               if (&t->rcu_node_entry == rnp->boost_tasks)
+               if (&t->rcu_node_entry == rnp->boost_tasks) {
                        rnp->boost_tasks = np;
+                       if (rnp->boost_tasks == NULL)
+                               rnp->boost_tasks = rnp->exp_tasks;
+               }
                /* Snapshot/clear ->rcu_boost_mutex with rcu_node lock held. */
                if (t->rcu_boost_mutex) {
                        rbmp = t->rcu_boost_mutex;
@@ -593,7 +596,8 @@ static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
        /* In case root is being boosted and leaf was not. */
        raw_spin_lock(&rnp_root->lock); /* irqs already disabled */
        if (rnp_root->boost_tasks != NULL &&
-           rnp_root->boost_tasks != rnp_root->gp_tasks)
+           rnp_root->boost_tasks != rnp_root->gp_tasks &&
+           rnp_root->boost_tasks != rnp_root->exp_tasks)
                rnp_root->boost_tasks = rnp_root->gp_tasks;
        raw_spin_unlock(&rnp_root->lock); /* irqs still disabled */
 #endif /* #ifdef CONFIG_RCU_BOOST */