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>

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 19b61ac..e978845 100644 (file)
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -97,6 +97,7 @@ struct rcu_dynticks {
                                    /* # times non-lazy CBs posted to CPU. */
        unsigned long nonlazy_posted_snap;
                                    /* idle-period nonlazy_posted snapshot. */
+       int tick_nohz_enabled_snap; /* Previously seen value from sysfs. */
 #endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
 };
 

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index c28d255..3508000 100644 (file)
--- 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

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 8699978..66ff07f 100644 (file)
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -105,7 +105,7 @@ static ktime_t tick_init_jiffy_update(void)
 /*
  * NO HZ enabled ?
  */
-static int tick_nohz_enabled __read_mostly  = 1;
+int tick_nohz_enabled __read_mostly  = 1;
 
 /*
  * Enable / Disable tickless mode