linux-kernel - Re: [PATCH V2 2/2] hrtimer: use __ffs() to iterate over valid bits from active

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Date:	Thu, 27 Mar 2014 06:40:58 +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 V2 2/2] hrtimer: use __ffs() to iterate over valid bits
 from active_bases

On Thu, 27 Mar 2014, Viresh Kumar wrote:

> Currently we are iterating over all possible (currently four) bits of
> active_bases to see if corresponding clock bases are active. This is good enough
> for cases where 3 or 4 bases are used but if only 1 or 2 are used then it makes
> more sense to use __ffs() to find the right bit directly.
> 
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> V1->V2: Instead of removing active_bases use __ffs() on it to make loop more
> efficient.
> 
> I tried to use for_each_set_bit() first and then it looked overdone. And so used
> a simple form, __ffs() with some code to clear bits.
> 
>  kernel/hrtimer.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
> index acfef5f..ea90228 100644
> --- a/kernel/hrtimer.c
> +++ b/kernel/hrtimer.c
> @@ -1265,6 +1265,7 @@ void hrtimer_interrupt(struct clock_event_device *dev)
>  {
>  	struct hrtimer_cpu_base *cpu_base = &__get_cpu_var(hrtimer_bases);
>  	ktime_t expires_next, now, entry_time, delta;
> +	unsigned long active_bases = cpu_base->active_bases;
>  	int i, retries = 0;
>  
>  	BUG_ON(!cpu_base->hres_active);
> @@ -1284,15 +1285,11 @@ retry:
>  	 */
>  	cpu_base->expires_next.tv64 = KTIME_MAX;
>  
> -	for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
> -		struct hrtimer_clock_base *base;
> +	while ((i = __ffs(active_bases))) {

What if this is a spurious interrupt and active_bases is 0?

Thanks,

	tglx
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