[PATCH v3] sched/idle: disable tick in idle=poll idle entry

[Marcelo Tosatti <mtosatti@redhat.com>]

Commit a5183862e76fdc25f36b39c2489b816a5c66e2e5
("tick/nohz: Conditionally restart tick on idle exit") allows
a nohz_full CPU to enter idle and return from it with the
scheduler tick disabled (since the tick might be undesired noise).

The idle=poll case still unconditionally restarts the tick when entering
idle.

To reduce the noise for that case as well, stop the tick when entering
idle, for the idle=poll case.

Change tick_nohz_full_kick_cpu to set NEED_RESCHED bit, to handle the
case where a new timer is added from an interrupt. This breaks out of
cpu_idle_poll and rearms the timer if necessary.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

---

v3: Add comment with proper explanation (Frederic Weisbecker)
    Add signed-off-by			(Thomas Gleixner)
v2: Handle the case where a new timer is added from an interrupt (Frederic Weisbecker)

 include/linux/sched.h    |    2 ++
 kernel/sched/core.c      |   10 ++++++++++
 kernel/sched/idle.c      |    2 +-
 kernel/time/tick-sched.c |    7 +++++++
 4 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index cbb7340c5866..1f6938dc20cd 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2428,4 +2428,6 @@ extern void migrate_enable(void);
 
 DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable())
 
+void set_tif_resched_if_polling(int cpu);
+
 #endif
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f1ebf67b48e2..f0b84600084b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -988,6 +988,11 @@ static bool set_nr_if_polling(struct task_struct *p)
 	return true;
 }
 
+void set_tif_resched_if_polling(int cpu)
+{
+	set_nr_if_polling(cpu_rq(cpu)->idle);
+}
+
 #else
 static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
 {
@@ -999,6 +1004,11 @@ static inline bool set_nr_if_polling(struct task_struct *p)
 {
 	return false;
 }
+
+void set_tif_resched_if_polling(int cpu)
+{
+	set_tsk_need_resched(cpu_rq(cpu)->idle);
+}
 #endif
 
 static bool __wake_q_add(struct wake_q_head *head, struct task_struct *task)
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index c39b089d4f09..428c2d1cbd1b 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -324,7 +324,7 @@ static void do_idle(void)
 		 * idle as we know that the IPI is going to arrive right away.
 		 */
 		if (cpu_idle_force_poll || tick_check_broadcast_expired()) {
-			tick_nohz_idle_restart_tick();
+			tick_nohz_idle_stop_tick();
 			cpu_idle_poll();
 		} else {
 			cpuidle_idle_call();
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index c527b421c865..9ec51da49591 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -408,6 +408,13 @@ void tick_nohz_full_kick_cpu(int cpu)
 	if (!tick_nohz_full_cpu(cpu))
 		return;
 
+	/*
+	 * When idle=poll, with the tick disabled (therefore idle CPU looping
+	 * at cpu_idle_poll), if a new timer is added from an interrupt,
+	 * the cpu_idle_poll only exits when TIF_NEED_RESCHED gets set.
+	 */
+	set_tif_resched_if_polling(cpu);
+
 	irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu);
 }