[PATCH RT 36/39] timers: Reduce future __run_timers() latency for newly emptied list

[Steven Rostedt <rostedt@goodmis.org>]

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3.14.34-rt32-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

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

upstream commit: 16d937f880312e3f47157d4d6d6ebf7e61523378

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, if we just emptied the timer wheel, for example, by deleting
the last timer, 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 catches ->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>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/timer.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/timer.c b/kernel/timer.c
index 9cf70c562fac..380f125ad100 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -691,6 +691,7 @@ detach_expired_timer(struct timer_list *timer, struct tvec_base *base)
 	if (!tbase_get_deferrable(timer->base))
 		base->active_timers--;
 	base->all_timers--;
+	(void)catchup_timer_jiffies(base);
 }
 
 static int detach_if_pending(struct timer_list *timer, struct tvec_base *base,
@@ -706,6 +707,7 @@ static int detach_if_pending(struct timer_list *timer, struct tvec_base *base,
 			base->next_timer = base->timer_jiffies;
 	}
 	base->all_timers--;
+	(void)catchup_timer_jiffies(base);
 	return 1;
 }
 
-- 
2.1.4