[tip:core/rcu] rcu: Prevent needless ->gp_seq_needed update in __note_gp_changes()

[tip-bot for Zhang, Jun]

Commit-ID:  13dc7d0c7a2ed438f0ec8e9fb365a1256d87cf87
Gitweb:     https://git.kernel.org/tip/13dc7d0c7a2ed438f0ec8e9fb365a1256d87cf87
Author:     Zhang, Jun <jun.zhang@xxxxxxxxx>
AuthorDate: Wed, 19 Dec 2018 10:37:34 -0800
Committer:  Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
CommitDate: Fri, 25 Jan 2019 15:30:00 -0800

rcu: Prevent needless ->gp_seq_needed update in __note_gp_changes()

Currently, __note_gp_changes() checks to see if the rcu_node structure's
->gp_seq_needed is greater than or equal to that of the rcu_data
structure, and if so, updates the rcu_data structure's ->gp_seq_needed
field.  This results in a useless store in the case where the two fields
are equal.

This commit therefore carries out this store only in the case where the
rcu_node structure's ->gp_seq_needed is strictly greater than that of
the rcu_data structure.

Signed-off-by: "Zhang, Jun" <jun.zhang@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/88DC34334CA3444C85D647DBFA962C2735AD5F77@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 21775eebb8f0..9d0e2ac9356e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1758,7 +1758,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
 		zero_cpu_stall_ticks(rdp);
 	}
 	rdp->gp_seq = rnp->gp_seq;  /* Remember new grace-period state. */
-	if (ULONG_CMP_GE(rnp->gp_seq_needed, rdp->gp_seq_needed) || rdp->gpwrap)
+	if (ULONG_CMP_LT(rdp->gp_seq_needed, rnp->gp_seq_needed) || rdp->gpwrap)
 		rdp->gp_seq_needed = rnp->gp_seq_needed;
 	WRITE_ONCE(rdp->gpwrap, false);
 	rcu_gpnum_ovf(rnp, rdp);