Re: [PATCH] cputime.h seems to assume HZ==1000

[George Anzinger <george@mvista.com>]

Linus Torvalds wrote:
> 
> On Mon, 17 Jan 2005, Roland McGrath wrote:
> 
>>Shouldn't msecs mean msecs, not secs/HZ?
> 
> 
> Hmm, sure, but why go through "msecs" at all?
> 
> 
>>--- linux-2.6/include/asm-generic/cputime.h
>>+++ linux-2.6/include/asm-generic/cputime.h
>>@@ -35,8 +35,8 @@ typedef u64 cputime64_t;
>> /*
>>  * Convert cputime to seconds and back.
>>  */
>>-#define cputime_to_secs(__ct)		(jiffies_to_msecs(__ct) / HZ)
>>-#define secs_to_cputime(__secs)		(msecs_to_jiffies(__secs * HZ))
>>+#define cputime_to_secs(__ct)		(jiffies_to_msecs(__ct) / 1000)
>>+#define secs_to_cputime(__secs)		(msecs_to_jiffies(__secs * 1000))
> 
> 
> iow, why not
> 
> 	#define cputime_to_secs(jif)	((jif) / HZ)
> 	#define secs_to_cputime(sec)	((sec) * HZ)
> 
> which avoids double rounding issues etc.

If we care, the jiffies_to_msecs() code is in include/linux/jiffies.h just prior 
to other conversion code that does NOT make the assumtion that HZ is exact.  To 
be exact:

static inline long cputime_to_secs(unsigned long jif)
{
	int t;
	u64 result = (u64)jif * TICK_NSEC;
	t = do_div(result ,NSEC_PER_SEC);
	return (u32)result + t ? 1:0;      /* round up if not exact */
}
#define secs_to_cputime(sec) (((U64) sec * SEC_CONVERSION) >> SEC_JIFFIE_SC)

This last assumes sec worth of jiffies will actually fit in a long...

-- 
George Anzinger   george@mvista.com
High-res-timers:  http://sourceforge.net/projects/high-res-timers/