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From b46f8c74afdd30cd52bfdcc2231470a0bab04416 Mon Sep 17 00:00:00 2001
From: James Hogan <james.hogan@imgtec.com>
Date: Fri, 22 Apr 2016 18:22:45 +0100
Subject: clockevents: Retry programming min delta up to 10 times
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.
(picked https://patchwork.kernel.org/patch/8909491/)
Signed-off-by: James Hogan <james.hogan@imgtec.com>
---
kernel/time/clockevents.c | 26 +++++++++++++++++++-------
1 file changed, 19 insertions(+), 7 deletions(-)
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -281,16 +281,28 @@ static int clockevents_program_min_delta
{
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 */
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