timers: Reduce future __run_timers() latency for first add to empty list

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>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Tested-by: Mike Galbraith <bitbucket@online.de>

diff --git a/kernel/timer.c b/kernel/timer.c
index dfac34f7186f60faf6ee0b6ad7b7d6dd1ed699dd..0c638cf3d9d21ce066bc0873431fd6d11063cc21 100644 (file)
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -398,6 +398,7 @@ __internal_add_timer(struct tvec_base *base, struct timer_list *timer)
 
 static void internal_add_timer(struct tvec_base *base, struct timer_list *timer)
 {
+       (void)catchup_timer_jiffies(base);
        __internal_add_timer(base, timer);
        /*
         * Update base->active_timers and base->next_timer