linux-kernel - Re: Linux 3.1-rc9

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Date:	Mon, 17 Oct 2011 11:03:10 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Simon Kirby <sim@...tway.ca>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: Linux 3.1-rc9

On Mon, 17 Oct 2011, Ingo Molnar wrote:
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> For the record, i absolutely hate much of the other time related type 
> obfuscation we do as well.
> 
> For example the ktime_t obfuscation - we only do it to avoid a divide 
> on 32-bit architectures that cannot do fast 64/32 divisions ...
> 
> It makes the time code a *lot* less obvious than it could be.
> 
> I think we should use one flat u64 nanoseconds time type in the 
> kernel (preparing it with using KTIME_SCALAR on all architectures for 
> a release or so), used with very simple and obvious C arithmetics.

It'd be nice, but this simply will not fly.
 
> That simple time type could then trickle down as well: we could use 
> it everywhere in kernel code and limit the hodge-podge of ABI time 
> units to the syscall boundary. (and convert the internal time unit to 
> whatever ABI unit there is close to the syscall boundary)
> 
> There's a point where micro-optimized 32-bit support related 
> maintenance overhead (and the resulting loss of 
> robustness/flexibility) becomes too expensive IMO.

That's not a micro optimization, it's a massive performance hit if you
force those 32bit archs to do 64/32 all over the place.

Thanks,

	tglx
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