linux-kernel - Re: [patch 18/20] timer: Forward wheel clock whenever possible

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Date:	Mon, 13 Jun 2016 17:18:17 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Richard Cochran <richardcochran@...il.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>,
	Arjan van de Ven <arjan@...radead.org>, rt@...utronix.de
Subject: Re: [patch 18/20] timer: Forward wheel clock whenever possible

On Mon, 13 Jun 2016, Richard Cochran wrote:
> On Mon, Jun 13, 2016 at 08:41:04AM -0000, Thomas Gleixner wrote:
> > @@ -498,23 +500,27 @@ static void internal_add_timer(struct ti
> >  	__internal_add_timer(base, timer);
> >  
> >  	/*
> > -	 * Check whether the other CPU is in dynticks mode and needs
> > -	 * to be triggered to reevaluate the timer wheel.  We are
> > -	 * protected against the other CPU fiddling with the timer by
> > -	 * holding the timer base lock. This also makes sure that a
> > -	 * CPU on the way to stop its tick can not evaluate the timer
> > -	 * wheel.
> > -	 *
> > -	 * Spare the IPI for deferrable timers on idle targets though.
> > -	 * The next busy ticks will take care of it. Except full dynticks
> > -	 * require special care against races with idle_cpu(), lets deal
> > -	 * with that later.
> > -	 */
> > -	if (IS_ENABLED(CONFIG_NO_HZ_COMMON) && base->nohz_active) {
> > -		if (!(timer->flags & TIMER_DEFERRABLE) ||
> > -		    tick_nohz_full_cpu(base->cpu))
> > -			wake_up_nohz_cpu(base->cpu);
> > -	}
> > +	 * We might have to IPI the remote CPU if the base is idle and the
> > +	 * timer is not deferrable. If the other cpu is on the way to idle
> > +	 * then it can't set base->is_idle as we hold base lock.
> > +	 */
> > +	if (!IS_ENABLED(CONFIG_NO_HZ_COMMON) || !base->is_idle ||
> > +	    (timer->flags & TIMER_DEFERRABLE))
> > +		return;
> 
> The tests for is_idle and TIMER_DEFERRABLE are actually checking the
> same thing.  I was looking at the usage of base.is_idle, and it is
> only ever set for BASE_STD.
> 
> So the TIMER_DEFERRABLE test is redundant, but maybe this is on
> purpose?  Anyhow, it did leave me scratching my head.

You're right. is_idle makes the deferrable check obsolete. We can remove it,
but it needs a big fat comment ....

Thanks

	tglx