linux-kernel - Re: [PATCH 1/2] hrtimer: don't add clock-base to active

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Date:	Thu, 27 Mar 2014 06:39:30 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Viresh Kumar <viresh.kumar@...aro.org>
cc:	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org,
	fweisbec@...il.com, linaro-networking@...aro.org
Subject: Re: [PATCH 1/2] hrtimer: don't add clock-base to active_bases if
 already present



On Thu, 27 Mar 2014, Viresh Kumar wrote:

> If active_bases already has entry for a particular clock type, then we don't
> need to rewrite it while queuing a hrtimer.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> Initially I thought of doing this but then thought better remove active_bases
> completely and so didn't sent this one. Now it might find some place for itself
> :).
> 
>  kernel/hrtimer.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
> index da351ad..acfef5f 100644
> --- a/kernel/hrtimer.c
> +++ b/kernel/hrtimer.c
> @@ -864,8 +864,9 @@ static int enqueue_hrtimer(struct hrtimer *timer,
>  {
>  	debug_activate(timer);
>  
> +	if (!timerqueue_getnext(&base->active))
> +		base->cpu_base->active_bases |= 1 << base->index;
>  	timerqueue_add(&base->active, &timer->node);
> -	base->cpu_base->active_bases |= 1 << base->index;

The conditional is more expensive than actually doing the OR operation
at least on x86 as it results in a branch.

Thanks,

	tglx
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