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>

1 files changed, 22 insertions, 8 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index cb5879386a02..b4e8eb24a5f1 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -800,33 +800,47 @@ void synchronize_rcu_expedited(void)
         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);