Re: [RFC PATCH 03/30] cputime: Introduce nsecs_to_cputime64()

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

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

> This will be needed for the conversion of kernel stat to nsecs.
> 
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Tony Luck <tony.luck@intel.com>
> Cc: Wu Fengguang <fengguang.wu@intel.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.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.