Commit c310ce4d authored Oct 08, 2017 by Zhenzhong Duan Committed by Thomas Gleixner Oct 18, 2017

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

parent b93ab338

kernel/time/timer.c

+5 −2

Original line number	Diff line number	Diff line
		@@ -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;