Re: [PATCH 0/4] x86_64: Optimize percpu accesses

Jeremy Fitzhardinge
2008-07-25T23:27:19Z

Mike Travis wrote:
> This patchset provides the following:
>
> * x86_64: Cleanup setup_percpu by fixing some minor potential
> problems as well as add some debugging aids.
>
> * x86_64: Rebase per cpu variables to zero
>
> Rebase per cpu variables to zero in preparation for the following
> patch to fold the pda into the per cpu area.
>
> * x86_64: Fold pda into per cpu area
>
> Declare the pda as a per cpu variable. This will allow the per cpu
> variables to be accessible on the x86_64 using %gs as the base of
> the percpu areas for each cpu:
>
> %gs:per_cpu_xxxx
>
> * x86_64: Reference zero-based percpu variables offset from gs
>
> Actually implement the above operation for __get_cpu_var() and
> __put_cpu_var(). Since this is now a single instruction, we
> can remove the non-preemptible versions of x86_read_percpu()
> and x86_write_percpu().
>

No, I think you've misunderstood these calls.

get_cpu_var(x) evaluates to an lvalue of this cpu's 'x'. It disables
preemption, in the same manner as get_cpu().

put_cpu_var(x) does nothing more than re-enable preemption, to pair with
get_cpu_var().

__get_cpu_var(x) is the same as get_cpu_var, but it assumes that
preemption is already disabled. There is no __put_cpu_var().

The important point is that an expression like "__get_cpu_var(x) = foo"
does not evaluate to a single instruction, and is not preempt or
interrupt -atomic. That's the reason x86_X_percpu() exist, since
they're a single instruction in an asm. However, with %gs: based
addressing they can be easily unified.

J