linux-kernel - Re: [RFD 4/9] Make total

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Date:	Wed, 28 Sep 2011 12:29:49 -0300
From:	Glauber Costa <glommer@...allels.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Martin Schwidefsky <schwidefsky@...ibm.com>,
	<linux-kernel@...r.kernel.org>, <paul@...lmenage.org>,
	<lizf@...fujitsu.com>, <daniel.lezcano@...e.fr>,
	<jbottomley@...allels.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [RFD 4/9] Make total_forks per-cgroup

On 09/28/2011 09:53 AM, Peter Zijlstra wrote:
> On Wed, 2011-09-28 at 14:42 +0200, Martin Schwidefsky wrote:
>
>>> That is, am I missing some added value of all this cputime*() foo?
>>
>> C can do the math as long as the encoding of the cputime is simple enough.
>> Can we demand that a cputime value needs to be an integral type ?
>
> I'd like to think we can ;-)
>
>> What I did when I wrote all that stuff is to define cputime_t as a struct
>> that contains a single u64. That way I found all the places in the kernel
>> that used a cputime and could convert the code accordingly.
>
> Indeed, that makes it a non-simple type and breaks all the C arith bits.
>
>> My fear is that if the cputime_xxx operations are removed, code will
>> sneak in again that just uses an unsigned long instead of a cputime_t.
>> That would break any arch that requires something bigger than a u32 for
>> its cputime.
>
> Which is only a problem for 32bit archs, of which s390 is the only one
> that matters, right? Hurm,. could we do something with sparse? Lots of
> people run sparse.
>
Well, I think x86-32 is unlikely to ever really go away.
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