Re: [PATCH 0/8] x86, acpi: Move acpi_initrd_override() earlier.

[Toshi Kani]

Hello,

On Fri, 2013-08-23 at 09:04 -0400, Tejun Heo wrote:
> On Thu, Aug 22, 2013 at 04:17:41PM -0600, Toshi Kani wrote:
> > I am relatively new to Linux, so I am not a good person to elaborate
> > this.  From my experience on other OS, huge pages helped for the kernel,
> > but did not necessarily help user applications.  It depended on
> > applications, which were not niche cases.  But Linux may be different,
> > so I asked since you seemed confident.  I'd appreciate if you can point
> > us some data that endorses your statement.
> 
> We are talking about the kernel linear mapping which is created during
> early boot, so if it's available and useable there's no reason not to
> use it.  Exceptions would be earlier processors which didn't do 1G
> mappings or e820 maps with a lot of holes.  For CPUs used in NUMA
> configurations, the former has been history for a bit now.  Can't be
> sure about the latter but it'd be surprising for that to affect large
> amount of memory in the systems that are of interest here.  Ooh, that
> reminds me that we probably wanna go back to 1G + MTRR mapping under
> 4G.  We're currently creating a lot of mapping holes.

Thanks for the explanation.

> > My worry is that the code is unlikely tested with the special logic when
> > someone makes code changes to the page tables.  Such code can easily be
> > broken in future.
> 
> Well, I wouldn't consider flipping the direction of allocation to be
> particularly difficult to get right especially when compared to
> bringing in ACPI tables into the mix.
> 
> > To answer your other question/email, I believe Tang's next step is to
> > support local page tables.  This is why we think pursing SRAT earlier is
> > the right direction.
> 
> Given 1G mappings, is that even a worthwhile effort?  I'm getting even
> more more skeptical.

With 1G mappings, I agree that it won't make much difference.

I still think acpi table info should be available earlier, but I do not
think I can convince you on this.  This can be religious debate.

Tang, what do you think?  Are you OK to try Tejun's suggestion as well? 

Thanks,
-Toshi

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