Re: [PATCH 3/9] ia64: VIRT_CPU_ACCOUNTING (accurate cpu time accounting)

Simon Horman
2007-10-30T02:41:00Z

On Tue, Oct 30, 2007 at 11:25:55AM +0900, Hidetoshi Seto wrote:
> Simon Horman wrote:
> > On Tue, Oct 16, 2007 at 10:35:52PM +0900, Hidetoshi Seto wrote:
> >> +#define cputime_to_jiffies(__ct) ((__ct) * HZ / NSEC_PER_SEC)
> >> +#define jiffies_to_cputime(__jif) ((__jif) * NSEC_PER_SEC / HZ)
> >> +#define cputime64_to_jiffies64(__ct) ((__ct) * HZ / NSEC_PER_SEC)
> >> +#define jiffies64_to_cputime64(__jif) ((__jif) * NSEC_PER_SEC / HZ)
> >
> > It looks like cputime64_to_jiffies64 and cputime_to_jiffies will
> > overflow at (((2^64 -1) / HZ) + 1) ns. In the case where HZ is 1000,
> > this means it will overflow at (584/1000) years or about 213 days.
> > Similarly for cputime_to_clock_t(). Is this a problem?
>
> Exactly.
> I guess NSEC_PER_SEC is permanently 10^9, and larger HZ will not be
> required soon in this century. Does the following help us?
>
> #define cputime64_to_jiffies64(__ct) ((__ct) / (NSEC_PER_SEC / HZ))
> #define jiffies64_to_cputime64(__jif) ((__jif) * (NSEC_PER_SEC / HZ))

That does seem to be a lot better. I guess there might be some rounding
strangeness if NSEC_PER_SEC is not an integer multiple of HZ, but
as both cputime64_to_jiffies64() and jiffies64_to_cputime64() use
the same rounded value, this shouldn't be a problem. Actually its
probably not a problem at all.

I guess there is also now an assumption that HZ is much less than
NSEC_PER_SEC, that also seems reasonable.

--
Horms
H: http://www.vergenet.net/~horms/
W: http://www.valinux.co.jp/en/