nohz: Re-evaluate the tick from the scheduler IPI

The scheduler IPI is used by the scheduler to kick
full dynticks CPUs asynchronously when more than one
task are running or when a new timer list timer is
enqueued. This way the destination CPU can decide
to restart the tick to handle this new situation.

Now let's call that kick in the scheduler IPI.

(Reusing the scheduler IPI rather than implementing
a new IPI was suggested by Peter Zijlstra a while ago)

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

include/linux/tick.h

@@ -160,11 +160,13 @@ static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
 #ifdef CONFIG_NO_HZ_FULL
 extern void tick_nohz_init(void);
 extern int tick_nohz_full_cpu(int cpu);
+extern void tick_nohz_full_check(void);
 extern void tick_nohz_full_kick(void);
 extern void tick_nohz_full_kick_all(void);
 #else
 static inline void tick_nohz_init(void) { }
 static inline int tick_nohz_full_cpu(int cpu) { return 0; }
+static inline void tick_nohz_full_check(void) { }
 static inline void tick_nohz_full_kick(void) { }
 static inline void tick_nohz_full_kick_all(void) { }
 #endif

kernel/sched/core.c

@@ -1398,7 +1398,8 @@ static void sched_ttwu_pending(void)
 
 void scheduler_ipi(void)
 {
-	if (llist_empty(&this_rq()->wake_list) && !got_nohz_idle_kick())
+	if (llist_empty(&this_rq()->wake_list) && !got_nohz_idle_kick()
+	    && !tick_nohz_full_cpu(smp_processor_id()))
 		return;
 
 	/*
@@ -1415,6 +1416,7 @@ void scheduler_ipi(void)
 	 * somewhat pessimize the simple resched case.
 	 */
 	irq_enter();
+	tick_nohz_full_check();
 	sched_ttwu_pending();
 
 	/*

kernel/time/tick-sched.c

@@ -151,7 +151,7 @@ bool have_nohz_full_mask;
  * Re-evaluate the need for the tick on the current CPU
  * and restart it if necessary.
  */
-static void tick_nohz_full_check(void)
+void tick_nohz_full_check(void)
 {
 	/*
 	 * STUB for now, will be filled with the full tick stop/restart