[tip:timers/core] timers: Avoid an unnecessary iteration in __run_timers()

[tip-bot for Zhenzhong Duan <tipbot@zytor.com>]

Commit-ID:  c310ce4dcb9df9b2f1be82caff7dae609fe53f72
Gitweb:     https://git.kernel.org/tip/c310ce4dcb9df9b2f1be82caff7dae609fe53f72
Author:     Zhenzhong Duan <zhenzhong.duan@oracle.com>
AuthorDate: Sun, 8 Oct 2017 20:55:59 -0700
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Wed, 18 Oct 2017 15:29:33 +0200

timers: Avoid an unnecessary iteration in __run_timers()

If the base clock is behind jiffies in the soft irq expiry code then the
next timer is retrieved by get_next_timer_interrupt() to avoid incrementing
base clock one by one. If the next timer interrupt is past current jiffies
then the base clock is set to jiffies - 1. At the call site this is
incremented and another iteration through the expiry loop is executed which
checks empty hash buckets.

That's a pointless excercise because it's already known that the next timer
is past jiffies.

Set the base clock in that case to jiffies directly so it gets incremented
to jiffies + 1 at the call site resulting in immediate termination of the
expiry loop.

[ tglx: Massaged changelog and added comment to the code ]

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Joe Jin <joe.jin@oracle.com>
Cc: sboyd@codeaurora.org
Cc: Srinivas Reddy Eeda <srinivas.eeda@oracle.com>
Cc: john.stultz@linaro.org
Link: https://lkml.kernel.org/r/7086a857-f90c-4616-bbe8-f7696f21626c@default
---
 kernel/time/timer.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 38613ce..ee1a88d 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1560,8 +1560,11 @@ static int collect_expired_timers(struct timer_base *base,
 		 * jiffies, otherwise forward to the next expiry time:
 		 */
 		if (time_after(next, jiffies)) {
-			/* The call site will increment clock! */
-			base->clk = jiffies - 1;
+			/*
+			 * The call site will increment base->clk and then
+			 * terminate the expiry loop immediately.
+			 */
+			base->clk = jiffies;
 			return 0;
 		}
 		base->clk = next;