rcu: Make rcu_gp_cleanup() more accurately predict need for new GP

Currently, rcu_gp_cleanup() scans the rcu_node tree in order to reset
state to reflect the end of the grace period.  It also checks to see
whether a new grace period is needed, but in a number of cases, rather
than directly cause the new grace period to be immediately started, it
instead leaves the grace-period-needed state where various fail-safes
can find it.  This works fine, but results in higher contention on the
root rcu_node structure's ->lock, which is undesirable, and contention
on that lock has recently become noticeable.

This commit therefore makes rcu_gp_cleanup() immediately start a new
grace period if there is any need for one.

It is quite possible that it will later be necessary to throttle the
grace-period rate, but that can be dealt with when and if.

Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>

1 files changed, 0 insertions, 17 deletions

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 640ea927d8a4..313b77d9cf06 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1780,19 +1780,6 @@ static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
 	swake_up_all(sq);
 }
 
-/*
- * Set the root rcu_node structure's ->need_future_gp field
- * based on the sum of those of all rcu_node structures.  This does
- * double-count the root rcu_node structure's requests, but this
- * is necessary to handle the possibility of a rcu_nocb_kthread()
- * having awakened during the time that the rcu_node structures
- * were being updated for the end of the previous grace period.
- */
-static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
-{
-	need_future_gp_element(rnp, rnp->completed + 1) += nrq;
-}
-
 static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
 {
 	return &rnp->nocb_gp_wq[rnp->completed & 0x1];
@@ -2495,10 +2482,6 @@ static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
 {
 }
 
-static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
-{
-}
-
 static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
 {
 	return NULL;