On Mon, Sep 28, 2015 at 11:48:16AM -0700, Yunhong Jiang wrote:
> Currently, when a new timer added to timer wheel for a nohz_active CPU,
> the target CPU will always be waked up.
>
> In fact, if the new added timer is after the base->next_timer, we don't
> need wake up the target CPU since it will not change the sleep time. A
> lazy wake up is better in such scenario.
>
> I cooked a test scenario. On my 32 cores system, a driver on CPU 15
> continuous enqueues timer to CPU 8/9/10/11 with random expire and then
> checks the idle_calls difference after 10 seconds. Below data shows
> that lazy wake up do reduce the wakeup a lot.
>
> w/o Lazy w/ lazy
> CPU 8: 135 88
> CPU 9: 238 43
> CPU 10: 157 83
> CPU 11: 172 70
>
> Signed-off-by: Yunhong Jiang <yunhong.ji...@linux.intel.com>
> ---
> kernel/time/timer.c | 21 ++++++++++++++-------
> 1 file changed, 14 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index d3f5e92f722a..a039d9e6b55a 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -414,6 +414,8 @@ __internal_add_timer(struct tvec_base *base, struct
> timer_list *timer)
>
> static void internal_add_timer(struct tvec_base *base, struct timer_list
> *timer)
> {
> + bool kick_nohz = false;
> +
> /* Advance base->jiffies, if the base is empty */
> if (!base->all_timers++)
> base->timer_jiffies = jiffies;
> @@ -424,9 +426,17 @@ static void internal_add_timer(struct tvec_base *base,
> struct timer_list *timer)
> */
> if (!(timer->flags & TIMER_DEFERRABLE)) {
> if (!base->active_timers++ ||
> - time_before(timer->expires, base->next_timer))
> + time_before(timer->expires, base->next_timer)) {
> base->next_timer = timer->expires;
> - }
> + /*
> + * CPU in dynticks need reevaluate the timer wheel
> + * if newer timer added with next_timer updated.
> + */
> + if (base->nohz_active)
> + kick_nohz = true;
> + }
> + } else if (base->nohz_active && tick_nohz_full_cpu(base->cpu))
> + kick_nohz = true;
>
> /*
> * Check whether the other CPU is in dynticks mode and needs
> @@ -441,11 +451,8 @@ static void internal_add_timer(struct tvec_base *base,
> struct timer_list *timer)
> * require special care against races with idle_cpu(), lets deal
> * with that later.
> */
> - if (base->nohz_active) {
> - if (!(timer->flags & TIMER_DEFERRABLE) ||
> - tick_nohz_full_cpu(base->cpu))
> - wake_up_nohz_cpu(base->cpu);
> - }
> + if (kick_nohz)
> + wake_up_nohz_cpu(base->cpu);
> }
This patch makes sense. Thomas?
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