linux-kernel - Re: [RFC] [PATCH 1/1] hrtimers: Cache next hrtimer

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Date:	Fri, 28 Aug 2009 12:34:21 -0400
From:	Ashwin Chaugule <ashwinc@...eaurora.org>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: [RFC] [PATCH 1/1] hrtimers: Cache next hrtimer

Thomas Gleixner wrote:

> On Fri, 28 Aug 2009, Ashwin Chaugule wrote:
>   
>> Thomas Gleixner wrote:
>> Just didn't know the following could have the same effect. (base->offset is
>> confusing)
>>     
>
> It's simple. We have CLOCK_MONOTONIC and CLOCK_REALTIME. The internal
> time base is CLOCK_MONOTONIC. So we use base->offset to convert
> CLOCK_REALTIME to CLOCK_MONOTONIC. In case the timer is
> CLOCK_MONOTONIC we so the substraction as well, but it simply
> subtracts 0 :)
>  
>   
Cool ! 

> Gah. Looking at the patch with an awake brain makes me feel
> stupid. Working version below.
>   
Um. Same thing happened again. See below .. :)

> ---
> diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
> index 49da79a..380682b 100644
> --- a/kernel/hrtimer.c
> +++ b/kernel/hrtimer.c
> @@ -906,19 +906,28 @@ static void __remove_hrtimer(struct hrtimer *timer,
>  			     struct hrtimer_clock_base *base,
>  			     unsigned long newstate, int reprogram)
>  {
> -	if (timer->state & HRTIMER_STATE_ENQUEUED) {
> -		/*
> -		 * Remove the timer from the rbtree and replace the
> -		 * first entry pointer if necessary.
> -		 */
> -		if (base->first == &timer->node) {
> -			base->first = rb_next(&timer->node);
> -			/* Reprogram the clock event device. if enabled */
> -			if (reprogram && hrtimer_hres_active())
> +	ktime_t expires;
> +
> +	if (!(timer->state & HRTIMER_STATE_ENQUEUED))
> +		goto out;
> +
> +	/*
> +	 * Remove the timer from the rbtree and replace the first
> +	 * entry pointer if necessary.
> +	 */
> +	if (base->first == &timer->node) {
> +		base->first = rb_next(&timer->node);       
>   
> +		/* Reprogram the clock event device. if enabled */
> +		if (reprogram && hrtimer_hres_active()) {
>   

  			   timer->total_calls++;


> +			expires = ktime_sub(hrtimer_get_expires(timer),
> +					    base->offset);
> +			if (base->cpu_base->expires_next.tv64 == expires.tv64)
>   

                          {

                                   timer->cache_hits++;    

>  				hrtimer_force_reprogram(base->cpu_base);
>   
                          }

>  		}
> -		rb_erase(&timer->node, &base->active);
>  	}
> +
> +	rb_erase(&timer->node, &base->active);
> +out:
>  	timer->state = newstate;
>  }
>   
 

Avg startup time 26.4 (10 runs) same as last run.

total_calls is again equal to cache_hits ... 
Its been a while since I wrote my patch. I'll need to look deeper to see if I've done something more. B)
Just to make sure, I ran my patch again, I clearly see cache_hits is always less than total_calls.


Cheers,
Ashwin



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