rcu/nocb: Use build-time no-CBs check in rcu_do_batch()

Currently, rcu_do_batch() invokes rcu_segcblist_is_offloaded() each time
it needs to know whether the current CPU is a no-CBs CPU.  Given that it
is not possible to change the no-CBs status of a CPU after boot, and given
that it is not possible to even have no-CBs CPUs in CONFIG_RCU_NOCB_CPU=n
kernels, this per-callback invocation wastes CPU.  This commit therefore
created a const on-stack variable to allow this check to be done only
once per rcu_do_batch() invocation.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e5f30b364276367da9cd6165dca4727762c23463..16dabd6b36d72986345c13466e52aff28bd2ca67 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2087,6 +2087,8 @@ int rcutree_dead_cpu(unsigned int cpu)
 static void rcu_do_batch(struct rcu_data *rdp)
 {
        unsigned long flags;
+       const bool offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
+                              rcu_segcblist_is_offloaded(&rdp->cblist);
        struct rcu_head *rhp;
        struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
        long bl, count;
@@ -2128,12 +2130,11 @@ static void rcu_do_batch(struct rcu_data *rdp)
                 * Stop only if limit reached and CPU has something to do.
                 * Note: The rcl structure counts down from zero.
                 */
-               if (-rcl.len >= bl &&
-                   !rcu_segcblist_is_offloaded(&rdp->cblist) &&
+               if (-rcl.len >= bl && !offloaded &&
                    (need_resched() ||
                     (!is_idle_task(current) && !rcu_is_callbacks_kthread())))
                        break;
-               if (rcu_segcblist_is_offloaded(&rdp->cblist)) {
+               if (offloaded) {
                        WARN_ON_ONCE(in_serving_softirq());
                        local_bh_enable();
                        lockdep_assert_irqs_enabled();
@@ -2175,8 +2176,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
        rcu_nocb_unlock_irqrestore(rdp, flags);
 
        /* Re-invoke RCU core processing if there are callbacks remaining. */
-       if (!rcu_segcblist_is_offloaded(&rdp->cblist) &&
-           rcu_segcblist_ready_cbs(&rdp->cblist))
+       if (!offloaded && rcu_segcblist_ready_cbs(&rdp->cblist))
                invoke_rcu_core();
 }