When rcu_cbs_completed() is invoked on a non-root rcu_node structure,
it unconditionally assumes that two grace periods must complete before
the callbacks at hand can be invoked. This is overly conservative because
if that non-root rcu_node structure believes that no grace period is in
progress, and if the corresponding rcu_state structure's ->gpnum field
has not yet been incremented, then these callbacks may safely be invoked
after only one grace period has completed.
This change is required to permit grace-period start requests to use
funnel locking, which is in turn permitted to reduce root rcu_node ->lock
contention, which has been observed by Nick Piggin. Furthermore, such
contention will likely be increased by the merging of RCU-bh, RCU-preempt,
and RCU-sched, so it makes sense to take steps to decrease it.
This commit therefore improves the accuracy of rcu_cbs_completed() when
invoked on a non-root rcu_node structure as described above.
Reported-by: Nicholas Piggin <npig...@gmail.com>
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
---
kernel/rcu/tree.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2a734692a581..f5ca72f2ed43 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1642,6 +1642,21 @@ static unsigned long rcu_cbs_completed(struct rcu_state
*rsp,
return rnp->completed + 1;
/*
+ * If the current rcu_node structure believes that RCU is
+ * idle, and if the rcu_state structure does not yet reflect
+ * the start of a new grace period, then the next grace period
+ * will suffice. The memory barrier is needed to accurately
+ * sample the rsp->gpnum, and pairs with the second lock
+ * acquisition in rcu_gp_init(), which is augmented with
+ * smp_mb__after_unlock_lock() for this purpose.
+ */
+ if (rnp->gpnum == rnp->completed) {
+ smp_mb(); /* See above block comment. */
+ if (READ_ONCE(rsp->gpnum) == rnp->completed)
+ return rnp->completed + 1;
+ }
+
+ /*
* Otherwise, wait for a possible partial grace period and
* then the subsequent full grace period.
*/
--
2.5.2
