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

Currently, rcu_core() 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 repeated runtime invocation wastes CPU.  This commit
therefore created a const on-stack variable to allow this check to be
done only once per rcu_core() invocation.

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

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 16dabd6..1493927 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2302,6 +2302,8 @@ static __latent_entropy void rcu_core(void)
        unsigned long flags;
        struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
        struct rcu_node *rnp = rdp->mynode;
+       const bool offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
+                              rcu_segcblist_is_offloaded(&rdp->cblist);
 
        if (cpu_is_offline(smp_processor_id()))
                return;
@@ -2321,8 +2323,7 @@ static __latent_entropy void rcu_core(void)
 
        /* No grace period and unregistered callbacks? */
        if (!rcu_gp_in_progress() &&
-           rcu_segcblist_is_enabled(&rdp->cblist) &&
-           !rcu_segcblist_is_offloaded(&rdp->cblist)) {
+           rcu_segcblist_is_enabled(&rdp->cblist) && !offloaded) {
                local_irq_save(flags);
                if (!rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
                        rcu_accelerate_cbs_unlocked(rnp, rdp);
@@ -2332,8 +2333,7 @@ static __latent_entropy void rcu_core(void)
        rcu_check_gp_start_stall(rnp, rdp, rcu_jiffies_till_stall_check());
 
        /* If there are callbacks ready, invoke them. */
-       if (!rcu_segcblist_is_offloaded(&rdp->cblist) &&
-           rcu_segcblist_ready_cbs(&rdp->cblist) &&
+       if (!offloaded && rcu_segcblist_ready_cbs(&rdp->cblist) &&
            likely(READ_ONCE(rcu_scheduler_fully_active)))
                rcu_do_batch(rdp);