Subject: Re: [PATCH 6/6] optimize account_system_vtime

From: Michael Neuling <[email-masked]>
Date: Sat, 15 Dec 2007 13:10:14 +1100

In message <[email-masked]> you wrote:
> We have multiple calls to has_feature being inlined, but gcc can't
> be sure that the store via get_paca() doesn't alias the path to
> cur_cpu_spec->feature.
> 
> Reorder to put the calls to read_purr and read_spurr adjacent to each
> other.  To add a sense of consistency, reorder the remaining lines to
> perform parallel steps on purr and scaled purr of each line instead of
> calculating and then using one value before going on to the next.
> 
> In addition, we can tell gcc that no SPURR means no PURR.  The test is

This was suppose read "no PURR means no SPURR"?

> completely hidden in the PURR case, and in the !PURR case the second test
> is eliminated resulting in the simple register copy in the out-of-line
> branch.
> 
> Further, gcc sees get_paca()->system_time referenced several times and
> allocates a register to address it (shadowing r13) instead of caching its
> value.  Reading into a local varable saves the shadow of r13 and removes
> a potentially duplicate load (between the nested if and its parent).
> 
> Signed-off-by: Milton Miller <[email-masked]>
> ---
> The purr and spurr fields of the paca are only used in this c code,
> but system_time and user_time are also used in asm and I decided to
> leave all of these fields in the paca.
> 
> Index: kernel/arch/powerpc/kernel/time.c
> ===================================================================
> --- kernel.orig/arch/powerpc/kernel/time.c	2007-12-13 21:58:10.000000000 -
0600
> +++ kernel/arch/powerpc/kernel/time.c	2007-12-13 22:00:36.000000000 -0600
> @@ -219,7 +219,11 @@ static u64 read_purr(void)
>   */
>  static u64 read_spurr(u64 purr)
>  {
> -	if (cpu_has_feature(CPU_FTR_SPURR))
> +	/*
> +	 * cpus without PURR won't have a SPURR
> +	 * We already know the former when we use this, so tell gcc
> +	 */
> +	if (cpu_has_feature(CPU_FTR_PURR) && cpu_has_feature(CPU_FTR_SPURR))
>  		return mfspr(SPRN_SPURR);
>  	return purr;
>  }
> @@ -230,29 +234,30 @@ static u64 read_spurr(u64 purr)
>   */
>  void account_system_vtime(struct task_struct *tsk)
>  {
> -	u64 now, nowscaled, delta, deltascaled;
> +	u64 now, nowscaled, delta, deltascaled, sys_time;
>  	unsigned long flags;
>  
>  	local_irq_save(flags);
>  	now = read_purr();
> -	delta = now - get_paca()->startpurr;
> -	get_paca()->startpurr = now;
>  	nowscaled = read_spurr(now);
> +	delta = now - get_paca()->startpurr;
>  	deltascaled = nowscaled - get_paca()->startspurr;
> +	get_paca()->startpurr = now;
>  	get_paca()->startspurr = nowscaled;
>  	if (!in_interrupt()) {
>  		/* deltascaled includes both user and system time.
>  		 * Hence scale it based on the purr ratio to estimate
>  		 * the system time */
> +		sys_time = get_paca()->system_time;
>  		if (get_paca()->user_time)
> -			deltascaled = deltascaled * get_paca()->system_time /
> -			     (get_paca()->system_time + get_paca()->user_time);
> -		delta += get_paca()->system_time;
> +			deltascaled = deltascaled * sys_time /
> +			     (sys_time + get_paca()->user_time);
> +		delta += sys_time;
>  		get_paca()->system_time = 0;
>  	}
>  	account_system_time(tsk, 0, delta);
> -	get_paca()->purrdelta = delta;
>  	account_system_time_scaled(tsk, deltascaled);
> +	get_paca()->purrdelta = delta;

Reordering looks ok to me.  

These changes are going to conflict and probably need to be re-optimised
due to this patch in the mm tree.

http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.24-rc5/2.6.24-rc5-mm1/broken-out/taskstats-scaled-time-cleanup.patch

This moves the s/purrdelta out of the paca and into per-cpu variables.  

It's nothing that can't be merged, just flagging it as a future
conflict. 

Mikey