Re: [PATCH v6 0/2] x86: Implement fast refcount overflow protection

[Peter Zijlstra]

On Mon, Jul 24, 2017 at 04:38:06PM +1000, Michael Ellerman wrote:

> What I'm not entirely clear on is what the best trade off is in terms of
> overhead vs checks. The summary of behaviour between the fast and full
> versions you promised Ingo will help there I think.

That's something that's probably completely different for PPC than it is
for x86. Both because your primitive is LL/SC and thus the saturation
semantics we need a cmpxchg loop for are more natural in your case
anyway, and the fact that your LL/SC is horrendously slow in any case.

Also, I still haven't seen an actual benchmark where our cmpxchg loop
actually regresses anything, just a lot of yelling about potential
regressions :/