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

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Message-ID: <20151020224751.GB31289@jnakajim-build>
Date:	Tue, 20 Oct 2015 15:47:51 -0700
From:	Yunhong Jiang <yunhong.jiang@...ux.intel.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, viresh.kumar@...aro.org
Subject: Re: [PATCH] timer: Lazily wakup nohz CPU when adding new timer.

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?

Thanks
--jyh

>   
> Thanks,
> 
> 	tglx
> --
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