Messages in this thread | | | Date | Fri, 25 Jul 2008 16:26:54 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH 0/4] x86_64: Optimize percpu accesses |
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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
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