rcu: Allow RCU grace-period cleanup to be preempted

RCU grace-period cleanup is currently carried out with interrupts
disabled, which can result in excessive latency spikes on large systems
(many hundreds or thousands of CPUs).  This patch therefore makes the
RCU grace-period cleanup be preemptible, including voluntary preemption
points, which should eliminate those latency spikes.  Similar spikes from
forcing of quiescent states will be dealt with similarly by later patches.

Updated to replace uses of spin_lock_irqsave() with spin_lock_irq(), as
suggested by Peter Zijlstra.

Reported-by: Mike Galbraith <mgalbraith@suse.de>
Reported-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 52c3102dc5f7a987936c2e79b887519404454988..ddc6acc85d269ce425e4bbac08bc5681e756a502 100644 (file)
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1151,7 +1151,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
                 * completed.
                 */
                if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
-                       raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+                       raw_spin_unlock_irq(&rnp->lock);
 
                        /*
                         * Propagate new ->completed value to rcu_node
@@ -1160,14 +1160,13 @@ static int __noreturn rcu_gp_kthread(void *arg)
                         * to process their callbacks.
                         */
                        rcu_for_each_node_breadth_first(rsp, rnp) {
-                               /* irqs already disabled. */
-                               raw_spin_lock(&rnp->lock);
+                               raw_spin_lock_irq(&rnp->lock);
                                rnp->completed = rsp->gpnum;
-                               /* irqs remain disabled. */
-                               raw_spin_unlock(&rnp->lock);
+                               raw_spin_unlock_irq(&rnp->lock);
+                               cond_resched();
                        }
                        rnp = rcu_get_root(rsp);
-                       raw_spin_lock(&rnp->lock); /* irqs already disabled. */
+                       raw_spin_lock_irq(&rnp->lock);
                }
 
                rsp->completed = rsp->gpnum; /* Declare grace period done. */