rcu: Precompute RCU_FAST_NO_HZ timer offsets

When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick()
calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the
next wakeup time based on the timer wheels.  Only later, when actually
entering the idle loop, rcu_prepare_for_idle() will be invoked.  In some
cases, rcu_prepare_for_idle() will post timers to wake the CPU back up.
But all for naught: The next wakeup time for the CPU has already been
computed, and posting a timer afterwards does not force that wakeup
time to be recomputed.  This means that rcu_prepare_for_idle()'s have
no effect.

This is not a problem on a busy system because something else will wake
up the CPU soon enough.  However, on lightly loaded systems, the CPU
might stay asleep for a considerable length of time.  If that CPU has
a callback that the rest of the system is waiting on, the system might
run very slowly or (in theory) even hang.

This commit avoids this problem by having rcu_needs_cpu() give
tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU
to wake back up, which tick_nohz_stop_sched_tick() takes into account
when programming the CPU's wakeup time.  An alternative approach is
for rcu_prepare_for_idle() to use hrtimers instead of normal timers,
but timers are much more efficient than are hrtimers for frequently
and repeatedly posting and cancelling a given timer, which is exactly
what RCU_FAST_NO_HZ does.

Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>

1 files changed, 4 insertions, 2 deletions

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index adb5e5a38cae..854dc4c5c271 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -87,8 +87,9 @@ static inline void kfree_call_rcu(struct rcu_head *head,
 
 #ifdef CONFIG_TINY_RCU
 
-static inline int rcu_needs_cpu(int cpu)
+static inline int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)
 {
+	*delta_jiffies = ULONG_MAX;
 	return 0;
 }
 
@@ -96,8 +97,9 @@ static inline int rcu_needs_cpu(int cpu)
 
 int rcu_preempt_needs_cpu(void);
 
-static inline int rcu_needs_cpu(int cpu)
+static inline int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)
 {
+	*delta_jiffies = ULONG_MAX;
 	return rcu_preempt_needs_cpu();
 }