Messages in this thread | | | Date | Wed, 09 Jul 2008 10:51:46 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [RFC 00/15] x86_64: Optimize percpu accesses |
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Christoph Lameter wrote: > What is the point of the pda_X interface? It does not exist on 32 bit. > The pda wastes the GS segment register on a small memory area. This patchset > makes the GS segment usable to reach all of the per cpu area by placing > the pda into the per cpu area. Thus the pda_X interface becomes obsolete > and the 32 bit per cpu stuff becomes usable under 64 bit unifying both > architectures. >
I think we agree on the desired outcome. I just disagree with the path to getting there.
>> I think we should start devolving things out of the pda in the other >> direction: make a series where each patch takes a member of struct >> x8664_pda, converts it to a per-cpu variable (where possible, the same >> one that 32-bit uses), and updates all the references accordingly. When >> the pda is as empty as it can be, we can look at removing the >> pda-specific interfaces. >> > > This patchset places the whole x8664_pda structure into the per cpu area and makes the pda macros operate on the x8664_pda structure in the per cpu area. Not sure why you want to go through the churn of doing it for each object separately. >
No, it's not churn doing it object at a time. If you convert pda.pcurrent into a percpu current_task variable, then at one stroke you've 1) shrunk the pda, 2) unified with i386. If you go through the process of converting all the read_pda(pcurrent) references into x86_read_percpu(pda.pcurrent) then that's a pure churn patch. It doesn't get rid of the pda variable, it doesn't unify with i386. All it does is remove a reference to a macro which was fairly inoffensive in the first place.
Once the pda has shrunk as much as it can (which remove everything except stack_canary, I think), then remove all the X_pda macros, since there won't be any users anyway.
J
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