Messages in this thread | | | Date | Wed, 15 Nov 2006 20:06:06 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] i386-pda UP optimization |
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* Jeremy Fitzhardinge <jeremy@goop.org> wrote:
> Arjan van de Ven wrote: > > segment register accesses really are not cheap. > > Also really it'll be better to use the register userspace is not using, > > but we had that discussion before; could you remind me why you picked > > %gs in the first place? > > > > To leave open the possibility of using the compiler's TLS support in > the kernel for percpu. I also measured the cost of reloading %gs vs > %fs, and found no difference between reloading a null selector vs a > non-null selector.
what point would there be in using it? It's not like the kernel could make use of the thread keyword anytime soon (it would need /all/ architectures to support it) ... and the kernel doesnt mind how the current per_cpu() primitives are implemented, via assembly or via C. In any case, it very much matters to see the precise cost of having the pda selector value in %gs versus %fs.
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