From: Simon Horman <horms@verge.net.au>
To: linux-ia64@vger.kernel.org
Subject: Re: [PATCH 3/9] ia64: VIRT_CPU_ACCOUNTING (accurate cpu time accounting)
Date: Tue, 30 Oct 2007 02:41:00 +0000 [thread overview]
Message-ID: <20071030024047.GC5361@verge.net.au> (raw)
In-Reply-To: <4714BE38.60807@jp.fujitsu.com>
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/
prev parent reply other threads:[~2007-10-30 2:41 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-10-16 13:35 [PATCH 3/9] ia64: VIRT_CPU_ACCOUNTING (accurate cpu time accounting) Hidetoshi Seto
2007-10-30 1:40 ` Simon Horman
2007-10-30 2:25 ` Hidetoshi Seto
2007-10-30 2:41 ` Simon Horman [this message]
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