Messages in this thread | | | Date | Tue, 14 Jan 2014 19:48:28 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH tip/core/timers 1/3] timers: Reduce __run_timers() latency for empty list |
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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.
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