[RFC v6 18/23] clockevents: min delta increment: calculate min_delta_ns from ticks

Nicolai Stange
2016-09-09T20:18:39Z

The use of a clockevent device's ->min_delta_ns in the event programming
path hinders upcoming changes to the clockevent core making it NTP
correction aware: both, ->mult and ->min_delta_ns would need to get
updated as well as consumed atomically and we'd rather like to avoid any
locking here.

We already have got ->min_delta_ticks_adjusted which
- resembles the value of ->min_delta_ns
- and is guaranteed to be always >= the hardware's hard limit
->min_delta_ticks and thus, can be used w/o locking as we don't care
for small deviations.

In clockevents_increase_min_delta(), don't use ->min_delta_ns but
calculate it dynamically from ->min_delta_ticks_adjusted.

As clockevents_increase_min_delta() gets invoked only rarely, the
additional division should not be an issue from a performance standpoint.

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
---
kernel/time/clockevents.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index aa7b325..86d9f97 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -210,23 +210,26 @@ int clockevents_tick_resume(struct clock_event_device *dev)
*/
static int clockevents_increase_min_delta(struct clock_event_device *dev)
{
+ u64 min_delta_ns = cev_delta2ns(dev->min_delta_ticks_adjusted, dev,
+ false);
+
/* Nothing to do if we already reached the limit */
- if (dev->min_delta_ns >= MIN_DELTA_LIMIT) {
+ if (min_delta_ns >= MIN_DELTA_LIMIT) {
printk_deferred(KERN_WARNING
"CE: Reprogramming failure. Giving up\n");
dev->next_event.tv64 = KTIME_MAX;
return -ETIME;
}

- if (dev->min_delta_ns < 5000)
- dev->min_delta_ns = 5000;
+ if (min_delta_ns < 5000)
+ min_delta_ns = 5000;
else
- dev->min_delta_ns += dev->min_delta_ns >> 1;
+ min_delta_ns += min_delta_ns >> 1;

- if (dev->min_delta_ns > MIN_DELTA_LIMIT)
- dev->min_delta_ns = MIN_DELTA_LIMIT;
+ if (min_delta_ns > MIN_DELTA_LIMIT)
+ min_delta_ns = MIN_DELTA_LIMIT;

- dev->min_delta_ticks_adjusted = (unsigned long)((dev->min_delta_ns *
+ dev->min_delta_ticks_adjusted = (unsigned long)((min_delta_ns *
dev->mult) >> dev->shift);
dev->min_delta_ticks_adjusted = max(dev->min_delta_ticks_adjusted,
dev->min_delta_ticks);
@@ -234,7 +237,7 @@ static int clockevents_increase_min_delta(struct clock_event_device *dev)
printk_deferred(KERN_WARNING
"CE: %s increased min_delta_ns to %llu nsec\n",
dev->name ? dev->name : "?",
- (unsigned long long) dev->min_delta_ns);
+ (unsigned long long) min_delta_ns);
return 0;
}

--
2.9.3