[PATCH 02/10] nohz: Restart nohz full tick from irq exit

[Frederic Weisbecker <fweisbec@gmail.com>]

Restart the tick when necessary from the irq exit path. It makes nohz
full more flexible, simplify the related IPIs and doesn't bring
significant overhead on irq exit.

In a longer term view, it will allow us to piggyback the nohz kick
on the scheduler IPI in the future instead of sending a dedicated IPI
that often doubles the scheduler IPI on task wakeup. This will require
more changes though including careful review of resched_curr() callers
to include nohz full needs.

Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
---
 include/linux/tick.h     |  8 --------
 kernel/time/tick-sched.c | 34 ++++++++++------------------------
 2 files changed, 10 insertions(+), 32 deletions(-)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 1ca93f2..7d35b0f 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -147,7 +147,6 @@ static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask)
 		cpumask_or(mask, mask, tick_nohz_full_mask);
 }
 
-extern void __tick_nohz_full_check(void);
 extern void tick_nohz_full_kick(void);
 extern void tick_nohz_full_kick_cpu(int cpu);
 extern void tick_nohz_full_kick_all(void);
@@ -156,7 +155,6 @@ extern void __tick_nohz_task_switch(struct task_struct *tsk);
 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_full_check(void) { }
 static inline void tick_nohz_full_kick_cpu(int cpu) { }
 static inline void tick_nohz_full_kick(void) { }
 static inline void tick_nohz_full_kick_all(void) { }
@@ -190,12 +188,6 @@ static inline void housekeeping_affine(struct task_struct *t)
 #endif
 }
 
-static inline void tick_nohz_full_check(void)
-{
-	if (tick_nohz_full_enabled())
-		__tick_nohz_full_check();
-}
-
 static inline void tick_nohz_task_switch(struct task_struct *tsk)
 {
 	if (tick_nohz_full_enabled())
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index d6c8eff..a06cd4a 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -197,25 +197,9 @@ static bool can_stop_full_tick(void)
 	return true;
 }
 
-static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now);
-
-/*
- * Re-evaluate the need for the tick on the current CPU
- * and restart it if necessary.
- */
-void __tick_nohz_full_check(void)
-{
-	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
-
-	if (tick_nohz_full_cpu(smp_processor_id())) {
-		if (ts->tick_stopped && !can_stop_full_tick())
-			tick_nohz_restart_sched_tick(ts, ktime_get());
-	}
-}
-
 static void nohz_full_kick_work_func(struct irq_work *work)
 {
-	__tick_nohz_full_check();
+	/* Empty, the tick restart happens on tick_nohz_irq_exit() */
 }
 
 static DEFINE_PER_CPU(struct irq_work, nohz_full_kick_work) = {
@@ -250,7 +234,7 @@ void tick_nohz_full_kick_cpu(int cpu)
 
 static void nohz_full_kick_ipi(void *info)
 {
-	__tick_nohz_full_check();
+	/* Empty, the tick restart happens on tick_nohz_irq_exit() */
 }
 
 /*
@@ -703,7 +687,9 @@ out:
 	return tick;
 }
 
-static void tick_nohz_full_stop_tick(struct tick_sched *ts)
+static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now);
+
+static void tick_nohz_full_update_tick(struct tick_sched *ts)
 {
 #ifdef CONFIG_NO_HZ_FULL
 	int cpu = smp_processor_id();
@@ -714,10 +700,10 @@ static void tick_nohz_full_stop_tick(struct tick_sched *ts)
 	if (!ts->tick_stopped && ts->nohz_mode == NOHZ_MODE_INACTIVE)
 		return;
 
-	if (!can_stop_full_tick())
-		return;
-
-	tick_nohz_stop_sched_tick(ts, ktime_get(), cpu);
+	if (can_stop_full_tick())
+		tick_nohz_stop_sched_tick(ts, ktime_get(), cpu);
+	else if (ts->tick_stopped)
+		tick_nohz_restart_sched_tick(ts, ktime_get());
 #endif
 }
 
@@ -847,7 +833,7 @@ void tick_nohz_irq_exit(void)
 	if (ts->inidle)
 		__tick_nohz_idle_enter(ts);
 	else
-		tick_nohz_full_stop_tick(ts);
+		tick_nohz_full_update_tick(ts);
 }
 
 /**
-- 
2.1.4