[PATCH] clockevents: Retry programming min delta up to 10 times

[James Hogan]

Under virtualisation it is possible to get unexpected latency during a
clockevent device's set_next_event() callback which can make it return
-ETIME even for a delta based on min_delta_ns.

The clockevents_program_min_delta() implementation for
CONFIG_GENERIC_CLOCKEVENTS_MIN_ADJUST=n doesn't handle retries when this
happens, nor does clockevents_program_event() or its callers when force
is true (for example hrtimer_reprogram()). This can result in hangs
until the clock event device does a full period.

It isn't appropriate to use MIN_ADJUST in this case as occasional
hypervisor induced high latency will cause min_delta_ns to quickly
increase to the maximum.

Instead, borrow the retry pattern from the MIN_ADJUST case, but without
making adjustments. We retry up to 10 times before giving up.

Signed-off-by: James Hogan <james.hogan@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
---
 kernel/time/clockevents.c | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index a9b76a40319e..35f445f1bf39 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -281,16 +281,28 @@ static int clockevents_program_min_delta(struct clock_event_device *dev)
 {
 	unsigned long long clc;
 	int64_t delta;
+	int i;
 
-	delta = dev->min_delta_ns;
-	dev->next_event = ktime_add_ns(ktime_get(), delta);
+	for (i = 0;;) {
+		delta = dev->min_delta_ns;
+		dev->next_event = ktime_add_ns(ktime_get(), delta);
 
-	if (clockevent_state_shutdown(dev))
-		return 0;
+		if (clockevent_state_shutdown(dev))
+			return 0;
 
-	dev->retries++;
-	clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
-	return dev->set_next_event((unsigned long) clc, dev);
+		dev->retries++;
+		clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
+		if (dev->set_next_event((unsigned long) clc, dev) == 0)
+			return 0;
+
+		if (++i > 9) {
+			/*
+			 * We tried 10 times to program the device with the
+			 * given min_delta_ns. Get out of here.
+			 */
+			return -ETIME;
+		}
+	}
 }
 
 #endif /* CONFIG_GENERIC_CLOCKEVENTS_MIN_ADJUST */
-- 
2.4.10