Messages in this thread | | | Date | Wed, 21 Oct 2015 16:16:31 +0530 | From | Viresh Kumar <> | Subject | Re: [PATCH] timer: Lazily wakup nohz CPU when adding new timer. |
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Cc'ing Frederic.
On 20-10-15, 15:47, Yunhong Jiang wrote: > On Sun, Oct 11, 2015 at 08:12:39PM +0200, Thomas Gleixner wrote: > > On Mon, 28 Sep 2015, Yunhong Jiang wrote: > > > 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; > > > > Why do you want to kick the other cpu when a deferrable timer got added? > > This is what happens in current implementation and this patch does not > change the logic. According to the comments, it's to avoid race with > idle_cpu(). Frankly speaking, I didn't get the idea of the race. > > Viresh, do you have any hints?
I haven't looked at the core since few months now and looks like I don't remember anything :)
This thread is where we discussed it initially: http://marc.info/?l=linux-kernel&m=139039035809125
AFAIU, this is why we kick the other CPU for a deferrable timer: - The other CPU is a full-dynticks capable CPU and may be running tickless and we should serve the timer in time (even if it is deferrable) if the CPU isn't idle. - We could have saved the kick for a full-dynticks idle CPU, but a race can happen where we thought the CPU is idle, but it has just started serving userspace tick-lessly. And the timer wouldn't be served for long time, even when the cpu was busy.
Ofcourse, Frederic will kick me if I forgot the lessons he gave me earlier :)
-- viresh
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