rcu: Avoid tick_nohz_active checks on NOCBs CPUs

Currently, rcu_prepare_for_idle() checks for tick_nohz_active, even on
individual NOCBs CPUs, unless all CPUs are marked as NOCBs CPUs at build
time.  This check is pointless on NOCBs CPUs because they never have any
callbacks posted, given that all of their callbacks are handed off to the
corresponding rcuo kthread.  There is a check for individually designated
NOCBs CPUs, but it pointelessly follows the check for tick_nohz_active.

This commit therefore moves the check for individually designated NOCBs
CPUs up with the check for CONFIG_RCU_NOCB_CPU_ALL.

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

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 42df93721e6f41a1dd298082a5f7ca422bf0bfd1..8e9d4a4d0326a6a57279d01c0205f137586b078d 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1513,7 +1513,8 @@ static void rcu_prepare_for_idle(void)
        struct rcu_state *rsp;
        int tne;
 
-       if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_ALL))
+       if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_ALL) ||
+           rcu_is_nocb_cpu(smp_processor_id()))
                return;
 
        /* Handle nohz enablement switches conservatively. */
@@ -1527,10 +1528,6 @@ static void rcu_prepare_for_idle(void)
        if (!tne)
                return;
 
-       /* If this is a no-CBs CPU, no callbacks, just return. */
-       if (rcu_is_nocb_cpu(smp_processor_id()))
-               return;
-
        /*
         * If a non-lazy callback arrived at a CPU having only lazy
         * callbacks, invoke RCU core for the side-effect of recalculating