rcu: Make RCU_FAST_NO_HZ account for pauses out of idle
Both Steven Rostedt's new idle-capable trace macros and the RCU_NONIDLE()
macro can cause RCU to momentarily pause out of idle without the rest
of the system being involved. This can cause rcu_prepare_for_idle()
to run through its state machine too quickly, which can in turn result
in needless scheduling-clock interrupts.
This commit therefore adds code to enable rcu_prepare_for_idle() to
distinguish between an initial entry to idle on the one hand (which needs
to advance the rcu_prepare_for_idle() state machine) and an idle reentry
due to idle-capable trace macros and RCU_NONIDLE() on the other hand
(which should avoid advancing the rcu_prepare_for_idle() state machine).
Additional state is maintained to allow the timer to be correctly reposted
when returning after a momentary pause out of idle, and even more state
is maintained to detect when new non-lazy callbacks have been enqueued
(which may require re-evaluation of the approach to idleness).
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_plugin.h b/kernel/rcutree_plugin.h
index 0f007b3..50c1797 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1938,6 +1938,14 @@
{
}
+/*
+ * Don't bother keeping a running count of the number of RCU callbacks
+ * posted because CONFIG_RCU_FAST_NO_HZ=n.
+ */
+static void rcu_idle_count_callbacks_posted(void)
+{
+}
+
#else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
/*
@@ -1981,6 +1989,10 @@
static DEFINE_PER_CPU(int, rcu_dyntick_drain);
static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
static DEFINE_PER_CPU(struct timer_list, rcu_idle_gp_timer);
+static DEFINE_PER_CPU(unsigned long, rcu_idle_gp_timer_expires);
+static DEFINE_PER_CPU(bool, rcu_idle_first_pass);
+static DEFINE_PER_CPU(unsigned long, rcu_nonlazy_posted);
+static DEFINE_PER_CPU(unsigned long, rcu_nonlazy_posted_snap);
/*
* Allow the CPU to enter dyntick-idle mode if either: (1) There are no
@@ -1993,6 +2005,8 @@
*/
int rcu_needs_cpu(int cpu)
{
+ /* Flag a new idle sojourn to the idle-entry state machine. */
+ per_cpu(rcu_idle_first_pass, cpu) = 1;
/* If no callbacks, RCU doesn't need the CPU. */
if (!rcu_cpu_has_callbacks(cpu))
return 0;
@@ -2096,6 +2110,26 @@
static void rcu_prepare_for_idle(int cpu)
{
/*
+ * If this is an idle re-entry, for example, due to use of
+ * RCU_NONIDLE() or the new idle-loop tracing API within the idle
+ * loop, then don't take any state-machine actions, unless the
+ * momentary exit from idle queued additional non-lazy callbacks.
+ * Instead, repost the rcu_idle_gp_timer if this CPU has callbacks
+ * pending.
+ */
+ if (!per_cpu(rcu_idle_first_pass, cpu) &&
+ (per_cpu(rcu_nonlazy_posted, cpu) ==
+ per_cpu(rcu_nonlazy_posted_snap, cpu))) {
+ if (rcu_cpu_has_callbacks(cpu))
+ mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
+ per_cpu(rcu_idle_gp_timer_expires, cpu));
+ return;
+ }
+ per_cpu(rcu_idle_first_pass, cpu) = 0;
+ per_cpu(rcu_nonlazy_posted_snap, cpu) =
+ per_cpu(rcu_nonlazy_posted, cpu) - 1;
+
+ /*
* If there are no callbacks on this CPU, enter dyntick-idle mode.
* Also reset state to avoid prejudicing later attempts.
*/
@@ -2127,11 +2161,15 @@
per_cpu(rcu_dyntick_drain, cpu) = 0;
per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
if (rcu_cpu_has_nonlazy_callbacks(cpu))
- mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
- jiffies + RCU_IDLE_GP_DELAY);
+ per_cpu(rcu_idle_gp_timer_expires, cpu) =
+ jiffies + RCU_IDLE_GP_DELAY;
else
- mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
- jiffies + RCU_IDLE_LAZY_GP_DELAY);
+ per_cpu(rcu_idle_gp_timer_expires, cpu) =
+ jiffies + RCU_IDLE_LAZY_GP_DELAY;
+ mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
+ per_cpu(rcu_idle_gp_timer_expires, cpu));
+ per_cpu(rcu_nonlazy_posted_snap, cpu) =
+ per_cpu(rcu_nonlazy_posted, cpu);
return; /* Nothing more to do immediately. */
} else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
/* We have hit the limit, so time to give up. */
@@ -2171,6 +2209,17 @@
trace_rcu_prep_idle("Callbacks drained");
}
+/*
+ * Keep a running count of callbacks posted so that rcu_prepare_for_idle()
+ * can detect when something out of the idle loop posts a callback.
+ * Of course, it had better do so either from a trace event designed to
+ * be called from idle or from within RCU_NONIDLE().
+ */
+static void rcu_idle_count_callbacks_posted(void)
+{
+ __this_cpu_add(rcu_nonlazy_posted, 1);
+}
+
#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
#ifdef CONFIG_RCU_CPU_STALL_INFO