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

From: Hidetoshi Seto <seto.hidetoshi_at_jp.fujitsu.com>
Date: 2007-10-30 13:25:55
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))

Thanks,
H.Seto
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Received on Tue Oct 30 13:26:21 2007

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