[PATCH tip/core/timers 3/3] timers: Reduce future __run_timers() latency for first add to empty list

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

The __run_timers() function currently steps through the list one jiffy at
a time in order to update the timer wheel.  However, if the timer wheel
is empty, no adjustment is needed other than updating ->timer_jiffies.
Therefore, just before we add a timer to an empty timer wheel, we should
mark the timer wheel as being up to date.  This marking will reduce (and
perhaps eliminate) the jiffy-stepping that a future __run_timers() call
will need to do in response to some future timer posting or migration.
This commit therefore updates ->timer_jiffies for this case.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/timer.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/timer.c b/kernel/timer.c
index 2b2e235ea7f1..e539dbf5b9c5 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -745,6 +745,7 @@ __mod_timer(struct timer_list *timer, unsigned long expires,
 
 	base = lock_timer_base(timer, &flags);
 
+	(void)catchup_timer_jiffies(base);
 	ret = detach_if_pending(timer, base, false);
 	if (!ret && pending_only)
 		goto out_unlock;
-- 
1.8.1.5