RE: [BISECTION RESULT] sched: revert cpu_clock topre-27ec4407790d075c325e1f4da0a19c56953cce23 state

[Luck, Tony]

> Can you guys on IA64 possibly set ar.k3 simply to zero or to some
> other similar value which cancels out the per-cpu computation?
>
> That's what sparc64 and other platforms do.

In cpu_init() we set ar.k3 to the base physical address of the percpu
area.  The alt-dtlb miss handler uses this register to set up a
TLB mapping from virtual address 0xffffffffffff0000 to this physical
address.

This means that most per-cpu accesses are very cheap (as the compiler
can use a small -ve offset from register r0 to load the virtual
address).

Accessing ar.k3 is a bit slow ... so making the per-cpu code
use that on every access to a per-cpu variable would be unpleasant.

-Tony
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