Messages in this thread | | | Date | Wed, 21 Nov 2007 11:01:43 -0800 (PST) | From | Christoph Lameter <> | Subject | Re: [rfc 08/45] cpu alloc: x86 support |
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On Wed, 21 Nov 2007, Andi Kleen wrote:
> The whole mapping for all CPUs cannot fit into 2GB of course, but the reference > linker managed range can.
Ok so you favor the solution where we subtract smp_processor_id() << shift?
> > The offset relative to %gs cannot be used if you have a loop and are > > calculating the addresses for all instances. That is what we are talking > > about. The CPU_xxx operations that are using the %gs register are fine and > > are not affected by the changes we are discussing. > > Sure it can -- you just get the base address from a global array > and then add the offset
Ok so generalize the data_offset for that case? I noted that other arches and i386 have a similar solution there. I fiddled around some more and found that the overhead that the subtraction introduces is equivalent to loading an 8 byte constant of the base.
Keeping the usage of data_offset can avoid the shift and the add for the __get_cpu_var case that needs CPU_PTR( ..., smp_processor_id()) because the load from data_offset avoid the shifting and adding of smp_processor_id().
For the loops this is not useful since the compiler can move the loading of the base pointer outside of the loop )if CPU_PTR needs to load an 8 byte constant pointers).
With loading the 8 byte base the loops actually become:
sum = 0 ptr = CPU_AREA_BASE while base < NR_CPUS << shift { sum = *ptr ptr += 1 << shift }
So I think we need to go with the implementation where CPU_PTR(var, cpu) is
CPU_AREA_BASE + cpu << shift + var_offset
The CPU_AREA_BASE will be loaded into a register. The var_offset usually ends up being an offset in a mov instruction.
> > > > > Then the reference data would be initdata and eventually freed. > > > That is similar to how the current per cpu data works. > > > > Yes that is also how the current patchset works. I just do not understand > > what you want changed. > > Anyways i think your current scheme cannot work (too much VM, placed at the wrong > place; some wrong assumptions).
The constant pointer solution fixes that. No need to despair.
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