rcu: Make RCU_FAST_NO_HZ respect nohz= boot parameter

If the nohz= boot parameter disables nohz, then RCU_FAST_NO_HZ needs to
also disable itself.  This commit therefore checks for tick_nohz_enabled
being zero, disabling rcu_prepare_for_idle() if so.  This commit assumes
that tick_nohz_enabled can change at runtime: If this is not the case,
then a simpler approach suffices.

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

1 files changed, 15 insertions, 0 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index c28d25522a81..3508000e077d 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1971,6 +1971,8 @@ static void rcu_idle_count_callbacks_posted(void)
 #define RCU_IDLE_GP_DELAY 4             /* Roughly one grace period. */
 #define RCU_IDLE_LAZY_GP_DELAY (6 * HZ) /* Roughly six seconds. */
 
+extern int tick_nohz_enabled;
+
 /*
  * Does the specified flavor of RCU have non-lazy callbacks pending on
  * the specified CPU?  Both RCU flavor and CPU are specified by the
@@ -2112,6 +2114,7 @@ static void rcu_cleanup_after_idle(int cpu)
 
         del_timer(&rdtp->idle_gp_timer);
         trace_rcu_prep_idle("Cleanup after idle");
+        rdtp->tick_nohz_enabled_snap = ACCESS_ONCE(tick_nohz_enabled);
 }
 
 /*
@@ -2137,6 +2140,18 @@ static void rcu_prepare_for_idle(int cpu)
 {
         struct timer_list *tp;
         struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
+        int tne;
+
+        /* Handle nohz enablement switches conservatively. */
+        tne = ACCESS_ONCE(tick_nohz_enabled);
+        if (tne != rdtp->tick_nohz_enabled_snap) {
+                if (rcu_cpu_has_callbacks(cpu))
+                        invoke_rcu_core(); /* force nohz to see update. */
+                rdtp->tick_nohz_enabled_snap = tne;
+                return;
+        }
+        if (!tne)
+                return;
 
         /*
          * If this is an idle re-entry, for example, due to use of