Re: [PATCH 01/16] math128: Introduce various 128bit primitives

[Ingo Molnar]

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Thu, 2012-10-25 at 15:26 -0700, Linus Torvalds wrote:
> > it's the *rest* of the "u128" math I really object to. I also wonder
> > about the u64xu64 math case for SCHED_DEADLINE, because I assume that
> > it doesn't actually end up using the 128-bit result in that form, but
> > scales it down again some way? 
> 
> No, it does a compare on two u128, so it doesn't loose any 
> precision. If it were to scale down again and loose precision 
> I'd agree with you that introducing the u128 stuff is 
> pointless.
> 
> The point is (as mentioned in the comments below) overflowing 
> an actual u64 is rare, however since some of this 
> (specifically the dl_{runtime,deadline} parameters) is user 
> specified, we have to assume we will overflow.

So can we control this by restricting the users and avoiding the 
overflow?

A 2^64 result should be a *huge* amount of space already for 
just about anything.

Thanks,

	Ingo
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