Re: Per-cpu patches on top of PDA stuff...

[Jeremy Fitzhardinge <jeremy@goop.org>]

Rusty Russell wrote:
> This patch uses the "gs" segment register which Jeremy Fitzhardinge
> freed up for kernel use, for the per-cpu section.  This means that
> instead of having a special per-cpu struct which we can access in a
> single instruction, any per-cpu variable can be accessed in a single
> instruction.  In addition, it avoids introducing the concept of a
> "pda" into the kernel, in favour of the well-known "percpu" concept.
>   

The PDA is well established in the x86-64 tree; one of the reasons for 
having an x86-32 PDA was to make the trees a bit more similar.

> More cleanups/optimizations are possible:
> 1) Don't save/restore %gs on UP.  The cost is measurable, and we don't use it.
>   

You need to make sure the userspace %gs gets saved/restored for context 
switch.  If you don't do it in entry.S, you need to have two versions of 
the %gs-handling code in a number of places around the kernel 
(__switch_to, vm86, ptrace).

> 2) Remove early_smp_processor_id(), by setting up the per-cpu
>    processor_id field correctly before starting a CPU.
> 3) Similarly, get rid of early_current().
>   

I've already submitted a patch to do this.  In fact, it would help if 
you could rebase against my most recent patches (which I think Andrew 
has queued for the next -mm, and are in the paravirt patch queue).

Also, does this patch address the problem of percpu module variables?  
That was the only real sticking point with my previous percpu-in-pda patch.

I can't say this patch fills me with joy.

    J