linux-kernel - Re: [RFC PATCH 07/30] cputime: Convert kcpustat to nsecs

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Date:	Mon, 1 Dec 2014 15:14:02 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: Re: [RFC PATCH 07/30] cputime: Convert kcpustat to nsecs

On Fri, 28 Nov 2014 19:23:37 +0100
Frederic Weisbecker <fweisbec@...il.com> wrote:

> Kernel cpu stats are stored in cputime_t which is an architecture
> defined type, and hence a bit opaque and requiring accessors and mutators
> for any operation.
> 
> Converting them to nsecs simplifies the code a little bit.

Quite honestly I do not see much of an improvement here, on set of
functions (cputime_to_xxx) gets replaced by another (nsecs_to_xxx).

On the contrary for s390 I see a degradation, consider a hunk like
this:

> @@ -128,9 +128,9 @@ static inline u64 get_cpu_idle_time_jiffy(unsigned int cpu, u64 *wall)
> 
>  	idle_time = cur_wall_time - busy_time;
>  	if (wall)
> -		*wall = cputime_to_usecs(cur_wall_time);
> +		*wall = div_u64(cur_wall_time, NSEC_PER_USEC);
> 
> -	return cputime_to_usecs(idle_time);
> +	return div_u64(idle_time, NSEC_PER_USEC);
>  }
> 
>  u64 get_cpu_idle_time(unsigned int cpu, u64 *wall, int io_busy)

For s390 cputime_to_usecs is a shift, with the new code we now have a division.
Fortunately this is in a piece of code that s390 does not use..

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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