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@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Link: https://lkml.kernel.org/r/88DC34334CA3444C85D647DBFA962C2735AD5F77@SHSMSX104.ccr.corp.intel.com

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 21775ee..9d0e2ac 100644 (file)
--- 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);