rcu: Reduce synchronize_rcu_expedited() latency

The synchronize_rcu_expedited() function disables interrupts across a
scan of all leaf rcu_node structures, which is not good for real-time
scheduling latency on large systems (hundreds or especially thousands
of CPUs).  This commit therefore holds off CPU-hotplug operations using
get_online_cpus(), and removes the prior acquisiion of the ->onofflock
(which required disabling interrupts).

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index cb5879386a022a933d85f4c58f8647e40e2b33c5..b4e8eb24a5f1584f2406f08b3e8e8c6850dc01a6 100644 (file)
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -799,34 +799,48 @@ void synchronize_rcu_expedited(void)
        snap = ACCESS_ONCE(sync_rcu_preempt_exp_count) + 1;
        smp_mb(); /* Above access cannot bleed into critical section. */
 
+       /*
+        * Block CPU-hotplug operations.  This means that any CPU-hotplug
+        * operation that finds an rcu_node structure with tasks in the
+        * process of being boosted will know that all tasks blocking
+        * this expedited grace period will already be in the process of
+        * being boosted.  This simplifies the process of moving tasks
+        * from leaf to root rcu_node structures.
+        */
+       get_online_cpus();
+
        /*
         * Acquire lock, falling back to synchronize_rcu() if too many
         * lock-acquisition failures.  Of course, if someone does the
         * expedited grace period for us, just leave.
         */
        while (!mutex_trylock(&sync_rcu_preempt_exp_mutex)) {
+               if (ULONG_CMP_LT(snap,
+                   ACCESS_ONCE(sync_rcu_preempt_exp_count))) {
+                       put_online_cpus();
+                       goto mb_ret; /* Others did our work for us. */
+               }
                if (trycount++ < 10) {
                        udelay(trycount * num_online_cpus());
                } else {
+                       put_online_cpus();
                        synchronize_rcu();
                        return;
                }
-               if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count)))
-                       goto mb_ret; /* Others did our work for us. */
        }
-       if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count)))
+       if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count))) {
+               put_online_cpus();
                goto unlock_mb_ret; /* Others did our work for us. */
+       }
 
        /* force all RCU readers onto ->blkd_tasks lists. */
        synchronize_sched_expedited();
 
-       raw_spin_lock_irqsave(&rsp->onofflock, flags);
-
        /* Initialize ->expmask for all non-leaf rcu_node structures. */
        rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) {
-               raw_spin_lock(&rnp->lock); /* irqs already disabled. */
+               raw_spin_lock_irqsave(&rnp->lock, flags);
                rnp->expmask = rnp->qsmaskinit;
-               raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+               raw_spin_unlock_irqrestore(&rnp->lock, flags);
        }
 
        /* Snapshot current state of ->blkd_tasks lists. */
@@ -835,7 +849,7 @@ void synchronize_rcu_expedited(void)
        if (NUM_RCU_NODES > 1)
                sync_rcu_preempt_exp_init(rsp, rcu_get_root(rsp));
 
-       raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
+       put_online_cpus();
 
        /* Wait for snapshotted ->blkd_tasks lists to drain. */
        rnp = rcu_get_root(rsp);