From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Wed, 4 Oct 2017 16:47:19 +0200
Subject: time/hrtimer: Use softirq based wakeups for non-RT threads
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: ddcf5a9f1152d0312538ee9ed5219d18d0318757
Patch-mainline: Queued in subsystem maintainer repository
References: SLE12 Realtime Extension

Normal wake ups (like clock_nanosleep()) which are performed by normal
users can easily lead to 2ms latency spikes if (enough) hrtimer wakeups
are synchronized.
This patch moves all hrtimers wakeups to the softirq queue unless the
caller has a RT priority.

Reported-by: Gratian Crisan <gratian.crisan@ni.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
---
 kernel/time/hrtimer.c |   21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1622,14 +1622,31 @@ static enum hrtimer_restart hrtimer_wake
 	return HRTIMER_NORESTART;
 }
 
+#ifdef CONFIG_PREEMPT_RT_FULL
+static bool task_is_elevated(struct task_struct *tsk)
+{
+	int policy = tsk->policy;
+
+	if (policy == SCHED_FIFO || policy == SCHED_RR)
+		return true;
+	if (policy == SCHED_DEADLINE)
+		return true;
+	return false;
+}
+#endif
+
 static void __hrtimer_init_sleeper(struct hrtimer_sleeper *sl,
 				   clockid_t clock_id,
 				   enum hrtimer_mode mode,
 				   struct task_struct *task)
 {
 #ifdef CONFIG_PREEMPT_RT_FULL
-	if (!(clock_id & HRTIMER_BASE_SOFT_MASK))
-		clock_id |= HRTIMER_BASE_HARD_MASK;
+	if (!(clock_id & (HRTIMER_BASE_HARD_MASK | HRTIMER_BASE_SOFT_MASK))) {
+		if (task_is_elevated(current) || system_state != SYSTEM_RUNNING)
+			clock_id |= HRTIMER_BASE_HARD_MASK;
+		else
+			clock_id |= HRTIMER_BASE_SOFT_MASK;
+	}
 #endif
 	__hrtimer_init(&sl->timer, clock_id, mode);
 	sl->timer.function = hrtimer_wakeup;