[PATCH rcu 4/4] rcu: Finer-grained grace-period-end checks in rcu_dump_cpu_stacks()

[Paul E. McKenney]

This commit pushes the grace-period-end checks further down into
rcu_dump_cpu_stacks(), and also uses lockless checks coupled with
finer-grained locking.

The result is that the current leaf rcu_node structure's ->lock is
acquired only if a stack backtrace might be needed from the current CPU,
and is held across only that CPU's backtrace.  As a result, if there are
no stalled CPUs associated with a given rcu_node structure, then its
->lock will not be acquired at all.  On large systems, it is usually
(though not always) the case that a small number of CPUs are stalling
the current grace period, which means that the ->lock need be acquired
only for a small fraction of the rcu_node structures.

Signed-off-by: Paul E. McKenney <paul...@kernel.org>
---
 kernel/rcu/tree_stall.h | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 9c8eb4b8dfb33..ab6848baba4f6 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -341,20 +341,24 @@ static void rcu_dump_cpu_stacks(unsigned long gp_seq)
        struct rcu_node *rnp;
 
        rcu_for_each_leaf_node(rnp) {
-               if (gp_seq != rcu_state.gp_seq) {
-                       pr_err("INFO: Stall ended during stack backtracing.\n");
-                       return;
-               }
                printk_deferred_enter();
-               raw_spin_lock_irqsave_rcu_node(rnp, flags);
-               for_each_leaf_node_possible_cpu(rnp, cpu)
+               for_each_leaf_node_possible_cpu(rnp, cpu) {
+                       if (gp_seq != data_race(rcu_state.gp_seq)) {
+                               printk_deferred_exit();
+                               pr_err("INFO: Stall ended during stack 
backtracing.\n");
+                               return;
+                       }
+                       if (!(data_race(rnp->qsmask) & leaf_node_cpu_bit(rnp, 
cpu)))
+                               continue;
+                       raw_spin_lock_irqsave_rcu_node(rnp, flags);
                        if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
                                if (cpu_is_offline(cpu))
                                        pr_err("Offline CPU %d blocking current 
GP.\n", cpu);
                                else
                                        dump_cpu_task(cpu);
+                       raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                        }
-               raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+               }
                printk_deferred_exit();
        }
 }
-- 
2.40.1