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Subject[PATCH tip/core/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 patch 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
--- 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
--- 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
--- 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


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