linux-kernel - Re: [PATCH tip/core/timers 2/4] timers: Reduce __run

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Date:	Wed, 15 Jan 2014 12:32:45 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu
Subject: Re: [PATCH tip/core/timers 2/4] timers: Reduce __run_timers()
 latency for empty list

On Wed, Jan 15, 2014 at 06:38:58PM +0100, Oleg Nesterov wrote:
> forgot to mention...
> 
> On 01/14, Paul E. McKenney wrote:
> >
> > +static bool catchup_timer_jiffies(struct tvec_base *base)
> > +{
> > +#ifdef CONFIG_NO_HZ_FULL
> > +	if (!base->all_timers) {
> > +		base->timer_jiffies = jiffies;
> > +		return 1;
> > +	}
> > +#endif /* #ifdef CONFIG_NO_HZ_FULL */
> > +	return 0;
> > +}
> 
> Do we really want ifdef?
> 
> This check is cheap, and !CONFIG_NO_HZ_FULL case looks a bit
> strange because we still update ->all_timers for no reason.

There's an easy way to improve this: instead of using an ifdef, put
"IS_ENABLED(CONFIG_NO_HZ_FULL) && " in the if condition.

- Josh Triplett
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