Re: [PATCH tip/core/timers 1/3] timers: Reduce __run_timers() latency for empty list

[Oleg Nesterov <oleg@redhat.com>]

On 01/13, Paul E. McKenney wrote:
>
> The __run_timers() function currently steps through the list one jiffy at
> a time in order to update the timer wheel.  However, if the timer wheel
> is empty, no adjustment is needed other than updating ->timer_jiffies.

Yes, but ->active_timers == 0 doesn't necessarily mean "empty", it only
counts the non-deferrable timers?

> In this case, which is likely to be common for NO_HZ_FULL kernels, the
> kernel currently incurs a large latency for no good reason.  This commit
> therefore short-circuits this case.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  kernel/timer.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/kernel/timer.c b/kernel/timer.c
> index 6582b82fa966..21849275828f 100644
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -337,6 +337,17 @@ void set_timer_slack(struct timer_list *timer, int slack_hz)
>  }
>  EXPORT_SYMBOL_GPL(set_timer_slack);
>
> +static bool catchup_timer_jiffies(struct tvec_base *base)
> +{
> +#ifdef CONFIG_NO_HZ_FULL
> +	if (!base->active_timers) {
> +		base->timer_jiffies = jiffies;
> +		return 1;
> +	}
> +#endif /* #ifdef CONFIG_NO_HZ_FULL */
> +	return 0;
> +}
> +
>  static void
>  __internal_add_timer(struct tvec_base *base, struct timer_list *timer)
>  {
> @@ -1146,6 +1157,10 @@ static inline void __run_timers(struct tvec_base *base)
>  	struct timer_list *timer;
>
>  	spin_lock_irq(&base->lock);

Do we really need to take base->lock before catchup_timer_jiffies() ?
->timer_jiffies can only be changed by us, and it seems that we do
not care if we race with base->active_timers++.

> +	if (catchup_timer_jiffies(base)) {
> +		spin_unlock_irq(&base->lock);
> +		return;

This is what I can't understand... Doesn't this mean that, unless this
base have a non-deferrable timer, we can never run the pending deferrable
timers even if the system/cpu is "busy" ?

Oleg.