Re: [PATCH v2] hrtimers: add fast path to hrtimer_get_next_event

[Viresh Kumar]

On 9 June 2014 18:29, Stanislav Fomichev <stfomichev@xxxxxxxxxxxxxx> wrote:
> In hrtimer_get_next_event we unconditionally lock/unlock spinlock, even if it's
> not required (hrtimer_hres_active() != 0). This patch adds fast path
> when highres is active so we don't execute unnecessary operations.
>
> We can safely do lockless check because:
> - hrtimer_get_next_event is always called with interrupts disabled;
> - we may switch to hres only from softirq handler with disabled interrupts.
>
> Because we only care about hres_active which may be changed only from
> local CPU, we can use interrupt context for synchronization.
>
> run_timer_softirq
>   hrtimer_run_pending
>     hrtimer_switch_to_hres
>       local_irq_save <-
>       base->hres_active = 0
>
> tick_nohz_idle_enter
>   local_irq_disable <-
>   __tick_nohz_idle_enter
>     tick_nohz_stop_sched_tick
>       get_next_timer_interrupt
>         cmp_next_hrtimer_event
>           hrtimer_get_next_event
>           check base->hres_active
>
> irq_exit <- irq context
>   tick_irq_exit
>     tick_nohz_irq_exit
>       tick_nohz_full_stop_tick
>         tick_nohz_stop_sched_tick
>             ... <see above>
>       __tick_nohz_idle_enter
>         ... <see above>
>
> Signed-off-by: Stanislav Fomichev <stfomichev@xxxxxxxxxxxxxx>
> ---
>  kernel/hrtimer.c | 27 ++++++++++++++-------------
>  1 file changed, 14 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
> index ba340739c701..731be91e927f 100644
> --- a/kernel/hrtimer.c
> +++ b/kernel/hrtimer.c
> @@ -1167,23 +1167,24 @@ ktime_t hrtimer_get_next_event(void)
>         unsigned long flags;
>         int i;
>
> +       if (hrtimer_hres_active())
> +               return mindelta;
> +
>         raw_spin_lock_irqsave(&cpu_base->lock, flags);
>
> -       if (!hrtimer_hres_active()) {
> -               for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++, base++) {
> -                       struct hrtimer *timer;
> -                       struct timerqueue_node *next;
> +       for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++, base++) {
> +               struct hrtimer *timer;
> +               struct timerqueue_node *next;
>
> -                       next = timerqueue_getnext(&base->active);
> -                       if (!next)
> -                               continue;
> +               next = timerqueue_getnext(&base->active);
> +               if (!next)
> +                       continue;
>
> -                       timer = container_of(next, struct hrtimer, node);
> -                       delta.tv64 = hrtimer_get_expires_tv64(timer);
> -                       delta = ktime_sub(delta, base->get_time());
> -                       if (delta.tv64 < mindelta.tv64)
> -                               mindelta.tv64 = delta.tv64;
> -               }
> +               timer = container_of(next, struct hrtimer, node);
> +               delta.tv64 = hrtimer_get_expires_tv64(timer);
> +               delta = ktime_sub(delta, base->get_time());
> +               if (delta.tv64 < mindelta.tv64)
> +                       mindelta.tv64 = delta.tv64;
>         }
>
>         raw_spin_unlock_irqrestore(&cpu_base->lock, flags);

Even I was concerned about a bigger diff earlier and so didn't comment
to rewrite it this way :)

Codewise, looks much better..

Reviewed-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
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