linux-kernel - Re: [RFC PATCH 03/30] cputime: Introduce nsecs_to

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Message-ID: <20141201150547.3521e8fa@mschwide>
Date:	Mon, 1 Dec 2014 15:05:47 +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 03/30] cputime: Introduce nsecs_to_cputime64()

On Fri, 28 Nov 2014 19:23:33 +0100
Frederic Weisbecker <fweisbec@...il.com> wrote:

> This will be needed for the conversion of kernel stat to nsecs.
> 
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Heiko Carstens <heiko.carstens@...ibm.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Tony Luck <tony.luck@...el.com>
> Cc: Wu Fengguang <fengguang.wu@...el.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> ---
>  include/linux/cputime.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/include/linux/cputime.h b/include/linux/cputime.h
> index f2eb2ee..a225ab9 100644
> --- a/include/linux/cputime.h
> +++ b/include/linux/cputime.h
> @@ -13,4 +13,14 @@
>  	usecs_to_cputime((__nsecs) / NSEC_PER_USEC)
>  #endif
> 
> +#ifndef nsecs_to_cputime
> +# define nsecs_to_cputime(__nsecs)	\
> +	usecs_to_cputime((__nsecs) / NSEC_PER_USEC)
> +#endif
> +
> +#ifndef nsecs_to_cputime64
> +# define nsecs_to_cputime64(__nsecs)	\
> +	((__force cputime64_t) nsecs_to_cputime(__nsecs))
> +#endif
> +
>  #endif /* __LINUX_CPUTIME_H */

For any architecture with a cputime_t better than a micro-second
the conversion to micro seconds degrades the precision a lot.

I would prefer to see the compile fail for e.g. s390 instead
of silently introducing *broken* cputime values.

-- 
blue skies,
   Martin.

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

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