Messages in this thread | | | Date | Wed, 28 Nov 2007 17:35:01 -0800 (PST) | From | Christoph Lameter <> | Subject | Re: [patch 05/14] percpu: Use a Kconfig variable to configure arch specific percpu setup |
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On Wed, 28 Nov 2007, Jeremy Fitzhardinge wrote:
> I don't see the problem. The way i386 does it inherently supports > per-cpu data very early on (it uses the prototype percpu section until > the real percpu values are set up).
Ok so we could do that for x86_64 as well? There is more complicated bootstrap since i386 does not support NUMA aware placement of per cpu areas.
> > The i386 way of referring to per cpu data is not optimal because it is > > always offset by __per_cpu_start. per cpu data offsets need to be relative > > to the beginning of the per cpu area. per cpu data is less than 64k so 2 > > byte offsets would be enough. > > > > I don't see that's terribly important. percpu references aren't all > that common overall, and - at least on x86 - using a 16-bit offset > (assuming its possible) would require a prefix anyway, so it would only > save 1 byte per reference. But I can't convince gas to generate a > 16-bit offset anyway.
percpu references are quite frequent already (vm statistics) and will be more frequent after we have converted the per cpu arrays to per cpu allocations.
> > That way the __per_cpu_offset array and the registers that are used on > > various platforms are pointing to the actual data and can be loaded > > directly into a register and then a load with a small offset to that > > register can be performed. On x86_64 this is gs, on i386 fs, on sparc g5, > > on ia64 a fixed address stands in for the register. > > The asm used to generate these references is inherently arch-specific > anyway, so the type and size of offset needed from the per-cpu base > register to the data itself can be arch-dependent without loss of > generality.
Well yes that is already the case and made explicit by the percpu cleanup done so far. The offset of a base is used by multiple architectures.
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