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

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 38613ced2324fce1c640484cbe9a7a7048ccdccc..ee1a88d8afb2cf3c629c626c82ff912a68f19471 100644 (file)
--- 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;