rcu: Micro-optimize rcu_cpu_has_callbacks()

The for_each_rcu_flavor() loop unconditionally scans all flavors, even
when the first flavor might have some non-lazy callbacks.  Once the
loop has seen a non-lazy callback, further passes through the loop
cannot change the state.  This is not a huge problem, given that there
can be at most three RCU flavors (RCU-bh, RCU-preempt, and RCU-sched),
but this code is on the path to idle, so speeding it up even a small
amount would have some benefit.

This commit therefore does two things:

1.	Rearranges the order of the list of RCU flavors in order to
	place the most active flavor first in the list.  The most active
	RCU flavor is RCU-preempt, or, if there is no RCU-preempt,
	RCU-sched.

2.	Reworks the for_each_rcu_flavor() to exit early when the first
	non-lazy callback is seen, or, in the case where the caller
	does not care about non-lazy callbacks (RCU_FAST_NO_HZ=n),
	when the first callback is seen.

Reported-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 7 insertions, 4 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index e6f2e8f14140..49464aded7f7 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2727,10 +2727,13 @@ static int rcu_cpu_has_callbacks(int cpu, bool *all_lazy)
 
         for_each_rcu_flavor(rsp) {
                 rdp = per_cpu_ptr(rsp->rda, cpu);
-                if (rdp->qlen != rdp->qlen_lazy)
+                if (!rdp->nxtlist)
+                        continue;
+                hc = true;
+                if (rdp->qlen != rdp->qlen_lazy || !all_lazy) {
                         al = false;
-                if (rdp->nxtlist)
-                        hc = true;
+                        break;
+                }
         }
         if (all_lazy)
                 *all_lazy = al;
@@ -3297,8 +3300,8 @@ void __init rcu_init(void)
 
         rcu_bootup_announce();
         rcu_init_geometry();
-        rcu_init_one(&rcu_sched_state, &rcu_sched_data);
         rcu_init_one(&rcu_bh_state, &rcu_bh_data);
+        rcu_init_one(&rcu_sched_state, &rcu_sched_data);
         __rcu_init_preempt();
         open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);