Re: [PATCH] timer: Lazily wakup nohz CPU when adding new timer.

[Viresh Kumar <viresh.kumar@linaro.org>]

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