nohz: Add TICK_DEP_BIT_RCU

If a nohz_full CPU is looping in the kernel, the scheduling-clock tick
might nevertheless remain disabled.  In !PREEMPT kernels, this can
prevent RCU's attempts to enlist the aid of that CPU's executions of
cond_resched(), which can in turn result in an arbitrarily delayed grace
period and thus an OOM.  RCU therefore needs a way to enable a holdout
nohz_full CPU's scheduler-clock interrupt.

This commit therefore provides a new TICK_DEP_BIT_RCU value which RCU can
pass to tick_dep_set_cpu() and friends to force on the scheduler-clock
interrupt for a specified CPU or task.  In some cases, rcutorture needs
to turn on the scheduler-clock tick, so this commit also exports the
relevant symbols to GPL-licensed modules.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/tick.h

@@ -108,14 +108,16 @@ enum tick_dep_bits {
 	TICK_DEP_BIT_POSIX_TIMER	= 0,
 	TICK_DEP_BIT_PERF_EVENTS	= 1,
 	TICK_DEP_BIT_SCHED		= 2,
-	TICK_DEP_BIT_CLOCK_UNSTABLE	= 3
+	TICK_DEP_BIT_CLOCK_UNSTABLE	= 3,
+	TICK_DEP_BIT_RCU		= 4
 };
 
 #define TICK_DEP_MASK_NONE		0
 #define TICK_DEP_MASK_POSIX_TIMER	(1 << TICK_DEP_BIT_POSIX_TIMER)
 #define TICK_DEP_MASK_PERF_EVENTS	(1 << TICK_DEP_BIT_PERF_EVENTS)
 #define TICK_DEP_MASK_SCHED		(1 << TICK_DEP_BIT_SCHED)
 #define TICK_DEP_MASK_CLOCK_UNSTABLE	(1 << TICK_DEP_BIT_CLOCK_UNSTABLE)
+#define TICK_DEP_MASK_RCU		(1 << TICK_DEP_BIT_RCU)
 
 #ifdef CONFIG_NO_HZ_COMMON
 extern bool tick_nohz_enabled;
@@ -268,6 +270,9 @@ static inline bool tick_nohz_full_enabled(void) { return false; }
 static inline bool tick_nohz_full_cpu(int cpu) { return false; }
 static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask) { }
 
+static inline void tick_nohz_dep_set_cpu(int cpu, enum tick_dep_bits bit) { }
+static inline void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit) { }
+
 static inline void tick_dep_set(enum tick_dep_bits bit) { }
 static inline void tick_dep_clear(enum tick_dep_bits bit) { }
 static inline void tick_dep_set_cpu(int cpu, enum tick_dep_bits bit) { }

include/trace/events/timer.h

@@ -367,7 +367,8 @@ TRACE_EVENT(itimer_expire,
 		tick_dep_name(POSIX_TIMER)		\
 		tick_dep_name(PERF_EVENTS)		\
 		tick_dep_name(SCHED)			\
-		tick_dep_name_end(CLOCK_UNSTABLE)
+		tick_dep_name(CLOCK_UNSTABLE)		\
+		tick_dep_name_end(RCU)
 
 #undef tick_dep_name
 #undef tick_dep_mask_name

kernel/time/tick-sched.c

@@ -198,6 +198,11 @@ static bool check_tick_dependency(atomic_t *dep)
 		return true;
 	}
 
+	if (val & TICK_DEP_MASK_RCU) {
+		trace_tick_stop(0, TICK_DEP_MASK_RCU);
+		return true;
+	}
+
 	return false;
 }
 
@@ -324,13 +329,15 @@ void tick_nohz_dep_set_cpu(int cpu, enum tick_dep_bits bit)
 		preempt_enable();
 	}
 }
+EXPORT_SYMBOL_GPL(tick_nohz_dep_set_cpu);
 
 void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit)
 {
 	struct tick_sched *ts = per_cpu_ptr(&tick_cpu_sched, cpu);
 
 	atomic_andnot(BIT(bit), &ts->tick_dep_mask);
 }
+EXPORT_SYMBOL_GPL(tick_nohz_dep_clear_cpu);
 
 /*
  * Set a per-task tick dependency. Posix CPU timers need this in order to elapse