[PATCH v16 05/13] Add try_stop_full_tick() API for NO_HZ_FULL

[Chris Metcalf]

This API checks to see if the scheduler tick can be stopped,
and if so, stops it and returns 0; otherwise it returns an error.
This is intended for use with task isolation, where we will want to
be able to stop the tick synchronously when returning to userspace.

Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
---
 include/linux/tick.h     |  1 +
 kernel/time/tick-sched.c | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index fe01e68bf520..078ff2464b00 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -234,6 +234,7 @@ static inline void tick_dep_clear_signal(struct signal_struct *signal,
 
 extern void tick_nohz_full_kick_cpu(int cpu);
 extern void __tick_nohz_task_switch(void);
+extern int try_stop_full_tick(void);
 #else
 static inline int housekeeping_any_cpu(void)
 {
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index c7a899c5ce64..c026145eba2f 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -861,6 +861,24 @@ static void tick_nohz_full_update_tick(struct tick_sched *ts)
 #endif
 }
 
+#ifdef CONFIG_TASK_ISOLATION
+int try_stop_full_tick(void)
+{
+	int cpu = smp_processor_id();
+	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
+
+	/* For an unstable clock, we should return a permanent error code. */
+	if (atomic_read(&tick_dep_mask) & TICK_DEP_MASK_CLOCK_UNSTABLE)
+		return -EINVAL;
+
+	if (!can_stop_full_tick(cpu, ts))
+		return -EAGAIN;
+
+	tick_nohz_stop_sched_tick(ts, ktime_get(), cpu);
+	return 0;
+}
+#endif
+
 static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
 {
 	/*
-- 
2.1.2