rcu: Make RCU_FAST_NO_HZ use timer rather than hrtimer

The RCU_FAST_NO_HZ facility uses an hrtimer to wake up a CPU when
it is allowed to go into dyntick-idle mode, which is almost always
cancelled soon after.  This is not what hrtimers are good at, so
this commit switches to the timer wheel.

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

kernel/rcutree_plugin.h

@@ -1980,9 +1980,7 @@ static void rcu_prepare_for_idle(int cpu)
 
 static DEFINE_PER_CPU(int, rcu_dyntick_drain);
 static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
-static DEFINE_PER_CPU(struct hrtimer, rcu_idle_gp_timer);
-static ktime_t rcu_idle_gp_wait;	/* If some non-lazy callbacks. */
-static ktime_t rcu_idle_lazy_gp_wait;	/* If only lazy callbacks. */
+static DEFINE_PER_CPU(struct timer_list, rcu_idle_gp_timer);
 
 /*
  * Allow the CPU to enter dyntick-idle mode if either: (1) There are no
@@ -2051,30 +2049,18 @@ static bool rcu_cpu_has_nonlazy_callbacks(int cpu)
  * real work is done upon re-entry to idle, or by the next scheduling-clock
  * interrupt should idle not be re-entered.
  */
-static enum hrtimer_restart rcu_idle_gp_timer_func(struct hrtimer *hrtp)
+static void rcu_idle_gp_timer_func(unsigned long unused)
 {
 	trace_rcu_prep_idle("Timer");
-	return HRTIMER_NORESTART;
 }
 
 /*
  * Initialize the timer used to pull CPUs out of dyntick-idle mode.
  */
 static void rcu_prepare_for_idle_init(int cpu)
 {
-	static int firsttime = 1;
-	struct hrtimer *hrtp = &per_cpu(rcu_idle_gp_timer, cpu);
-
-	hrtimer_init(hrtp, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
-	hrtp->function = rcu_idle_gp_timer_func;
-	if (firsttime) {
-		unsigned int upj = jiffies_to_usecs(RCU_IDLE_GP_DELAY);
-
-		rcu_idle_gp_wait = ns_to_ktime(upj * (u64)1000);
-		upj = jiffies_to_usecs(RCU_IDLE_LAZY_GP_DELAY);
-		rcu_idle_lazy_gp_wait = ns_to_ktime(upj * (u64)1000);
-		firsttime = 0;
-	}
+	setup_timer(&per_cpu(rcu_idle_gp_timer, cpu),
+		    rcu_idle_gp_timer_func, 0);
 }
 
 /*
@@ -2084,7 +2070,7 @@ static void rcu_prepare_for_idle_init(int cpu)
  */
 static void rcu_cleanup_after_idle(int cpu)
 {
-	hrtimer_cancel(&per_cpu(rcu_idle_gp_timer, cpu));
+	del_timer(&per_cpu(rcu_idle_gp_timer, cpu));
 	trace_rcu_prep_idle("Cleanup after idle");
 }
 
@@ -2141,11 +2127,11 @@ static void rcu_prepare_for_idle(int cpu)
 		per_cpu(rcu_dyntick_drain, cpu) = 0;
 		per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
 		if (rcu_cpu_has_nonlazy_callbacks(cpu))
-			hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
-				      rcu_idle_gp_wait, HRTIMER_MODE_REL);
+			mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
+					   jiffies + RCU_IDLE_GP_DELAY);
 		else
-			hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
-				      rcu_idle_lazy_gp_wait, HRTIMER_MODE_REL);
+			mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
+					   jiffies + RCU_IDLE_LAZY_GP_DELAY);
 		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. */
@@ -2193,14 +2179,12 @@ static void rcu_prepare_for_idle(int cpu)
 
 static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
 {
-	struct hrtimer *hrtp = &per_cpu(rcu_idle_gp_timer, cpu);
+	struct timer_list *tltp = &per_cpu(rcu_idle_gp_timer, cpu);
 
-	sprintf(cp, "drain=%d %c timer=%lld",
+	sprintf(cp, "drain=%d %c timer=%lu",
 		per_cpu(rcu_dyntick_drain, cpu),
 		per_cpu(rcu_dyntick_holdoff, cpu) == jiffies ? 'H' : '.',
-		hrtimer_active(hrtp)
-			? ktime_to_us(hrtimer_get_remaining(hrtp))
-			: -1);
+		timer_pending(tltp) ? tltp->expires - jiffies : -1);
 }
 
 #else /* #ifdef CONFIG_RCU_FAST_NO_HZ */