Re: [PATCH] i386-pda UP optimization

[Ingo Molnar]

* Jeremy Fitzhardinge <jeremy@xxxxxxxx> 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.

	Ingo
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