linux-kernel - Re: [patch 05/15] clocksource: Allow clocksource select to skip current clocksource

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Date:	Mon, 29 Apr 2013 18:00:27 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Magnus Damm <magnus.damm@...il.com>
Subject: Re: [patch 05/15] clocksource: Allow clocksource select to skip current
 clocksource

On 04/25/2013 01:31 PM, Thomas Gleixner wrote:
> Preparatory patch for clocksource unbind support.
>
> Modify clocksource_select, so it skips the current clocksource on
> request and tries to find a fallback clocksource. Convert all existing
> users. No functional change.
>
> Signed-off-by: Thomas Gleixner<tglx@...utronix.de>
> ---
>   kernel/time/clocksource.c |   24 ++++++++++++++----------
>   1 file changed, 14 insertions(+), 10 deletions(-)
>
> Index: tip/kernel/time/clocksource.c
> ===================================================================
> --- tip.orig/kernel/time/clocksource.c
> +++ tip/kernel/time/clocksource.c
> @@ -553,7 +553,7 @@ static u64 clocksource_max_deferment(str
>   
>   #ifndef CONFIG_ARCH_USES_GETTIMEOFFSET
>   
> -static struct clocksource *clocksource_find_best(bool oneshot)
> +static struct clocksource *clocksource_find_best(bool oneshot, bool skipcur)
>   {
>   	struct clocksource *cs;
>   
> @@ -566,6 +566,8 @@ static struct clocksource *clocksource_f
>   	 * the best rating.
>   	 */
>   	list_for_each_entry(cs, &clocksource_list, list) {
> +		if (skipcur && cs == curr_clocksource)
> +			continue;
>   		if (oneshot && !(cs->flags & CLOCK_SOURCE_VALID_FOR_HRES))
>   			continue;
>   		return cs;
> @@ -581,18 +583,20 @@ static struct clocksource *clocksource_f
>    * Select the clocksource with the best rating, or the clocksource,
>    * which is selected by userspace override.
>    */
> -static void clocksource_select(void)
> +static void clocksource_select(bool skipcur)
>   {
>   	bool oneshot = tick_oneshot_mode_active();
>   	struct clocksource *best, *cs;
>   
>   	/* Find the best suitable clocksource */
> -	best = clocksource_find_best(oneshot);
> +	best = clocksource_find_best(oneshot, skipcur);
>   	if (!best)
>   		return;
>   
>   	/* Check for the override clocksource. */
>   	list_for_each_entry(cs, &clocksource_list, list) {
> +		if (skipcur && cs == curr_clocksource)
> +			continue;
>   		if (strcmp(cs->name, override_name) != 0)
>   			continue;
>   		/*
> @@ -620,7 +624,7 @@ static void clocksource_select(void)
>   
>   #else /* !CONFIG_ARCH_USES_GETTIMEOFFSET */
>   
> -static inline void clocksource_select(void) { }
> +static inline void clocksource_select(bool skipcur) { }
>   
>   #endif

Not a major objection, but I sort of don't like the bool flag arguments 
to function that alters their behavior, since its not really very 
descriptive. I know I'll be looking at the code below thinking "hmmm.. 
clocksource_select(false).. now what does that mean?" at some point not 
too far away.

Instead, would leaving clocksource_select() alone, and introducing a 
clocksource_select_fallback() for the skipcur behavior, be a more 
descriptive choice? The code could very much use the same skipcur logic, 
in a __clocksource_select() or something function, but just helps make 
the normal usage easier to read.




>   
> @@ -645,7 +649,7 @@ static int __init clocksource_done_booti
>   	clocksource_watchdog_kthread(NULL);
>   
>   	mutex_lock(&clocksource_mutex);
> -	clocksource_select();
> +	clocksource_select(false);
>   	mutex_unlock(&clocksource_mutex);
>   	return 0;
>   }
> @@ -739,7 +743,7 @@ int __clocksource_register_scale(struct
>   	mutex_lock(&clocksource_mutex);
>   	clocksource_enqueue(cs);
>   	clocksource_enqueue_watchdog(cs);
> -	clocksource_select();
> +	clocksource_select(false);
>   	mutex_unlock(&clocksource_mutex);
>   	return 0;
>   }
> @@ -766,7 +770,7 @@ int clocksource_register(struct clocksou
>   	mutex_lock(&clocksource_mutex);
>   	clocksource_enqueue(cs);
>   	clocksource_enqueue_watchdog(cs);
> -	clocksource_select();
> +	clocksource_select(false);
>   	mutex_unlock(&clocksource_mutex);
>   	return 0;
>   }

thanks
-john

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