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

Mike Travis
2008-07-26T00:27:21Z

Jeremy Fitzhardinge wrote:
> 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

Yes, you're right, I wrote that quickly without really reading it back.
My point is that now that x86_read_percpu() and x86_write_percpu() do
evaluate to a single instruction (by definition atomic), then it doesn't
need to be surrounded by the preempt_disable()/preempt_enable() calls.

It appears as if I'm implying that's the case for get/put_cpu_var().

Thanks,
Mike