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:
> > 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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