Subject: Re: [PATCH] elapsed times wrap
From: Albert Cahalan <albert@users.sf.net>
To: linux-kernel@vger.kernel.org
Cc: hugh@veritas.com, kai@tp1.ruhr-uni-bochum.de
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Date: 24 Feb 2003 17:00:45 -0500
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Hugh Dickins writes:
> On Sat, 22 Feb 2003, Kai Germaschewski wrote:
>> On Sat, 22 Feb 2003, Hugh Dickins wrote:

>>> Userspace shows huge elapsed time across jiffies wrap:
>>> with USER_HZ less then HZ, sys_times needs jiffies_64
>>> to calculate its retval.
>>
>> That makes me wonder, aren't all uses of
>> jiffies_to_clock_t() broken then? 
>
> I believe you're right, but it's less obvious to me
> that the other uses really want fixing e.g. would we
> be happy to maintain utime,stime,cutime,cstime as
> 64-bit on a 32-bit machine?
>
>> Well, all which take an absolute time as an argument at least.
>
> Yes, it's much more important to fix those where userspace
> habitually takes the difference.  That certainly applies
> to the return value from sys_times, but I don't see any
> other cases as clear (though userspace may have good reason
> to take the difference of any of them).
>
> Perhaps a procps expert can advise?

That depends on how much you care about the problems.
Some that come to mind:

The OOM killer will be more likely to kill the wrong process.
CPU usage stats will be worthless junk.

On a 4-way box, you can hit troubles with cutime after
just 2 weeks of usage.

Consider changing just cutime. It's the value most likely
to wrap. Plain utime would be the second priority.


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