On Fri, 2006-09-22 at 14:32 +0200, Andi Kleen wrote:

BTW I changed my copy sorry. I redid the early PDA support
to not be in assembler.

On Fri, Sep 22, 2006 at 09:59:45PM +1000, Rusty Russell wrote:

quoted

This patch actually uses the gs register to implement the per-cpu
sections.  It's fairly straightforward: the gs segment starts at the
per-cpu offset for the particular cpu (or 0, in very early boot).  

We also implement x86_64-inspired (via Jeremy Fitzhardinge) per-cpu
accesses where a general lvalue isn't needed.  These
single-instruction accesses are slightly more efficient, plus (being a
single insn) are atomic wrt. preemption so we can use them to
implement cpu_local_inc etc.

The problem is nobody uses cpu_local_inc() etc :/ And it is difficult
to use in generic code because of the usual preemption issues 
(and being slower on other archs in many cases compared to preempt disabling
around larger block of code) 

True, they were primarily designed for the SNMP counters in networking
(as per your comment in snmp.h).  They're now dynamic per-cpu pointers,
which adds a new wrinkle though 8(

Without that it is the same code as Jeremy's variant
%gs memory reference + another reference with offset as far as I can see.

So while it looks nice I don't think it will have advantages. Or did
i miss something?

Mainly that it makes more sense to use the existing per-cpu concept than
introduce another kind of per-cpu var within a special structure, but
it's also more efficient (see other post).  Hopefully it will spark
interest in making dynamic-percpu pointers use the same offset scheme,
now x86 will experience the benefits.

And we might even get a third user of local_t!

Cheers,
Rusty.
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