Re: [RFC PATCH 00/09] robust VM per_cpu variables

[Steven Rostedt <rostedt@goodmis.org>]

Hi Christoph,

Thanks for replying!

On Wed, 17 May 2006, Christoph Lameter wrote:

> On Wed, 17 May 2006, Steven Rostedt wrote:
>
> > My first attempt to fix this introduced another dereference, to allow
> > for modules to allocate their own memory.  This was quickly shot down,
> > and for good reason, because dereferences kill performance, and don't
> > play nice with large SMP systems that depend on per_cpu being fast.
>
> > I now place the per_cpu variables into VM, such that the pages are
> > only allocated when needed. All the architecture needs to do is
> > supply a VM address range, size for each CPU to use (note this
> > implementation expects all the VM CPU areas to be together), and
> > three functions to allow for allocating page tables at bootup.
>
> So now instead of an explicit indirection we use an implicit one
> through the page tables for this. This happens during early boot which
> requires additional page table functions? And it requires the use of an
> additional TLB entry? I guess that the additional TLB pressure alone will
> result in a performance drop of 3%?

Ouch!

>
> See http://www.gelato.unsw.edu.au/archives/linux-ia64/0602/17311.html

Thanks for the link.

Hmm, my main goal is still to make the per_cpu more robust, so that the
generic code is truely that, and the hacks are better managed.  Would the
TLB pressure on a normal desktop also cause the drop in performance?  I
haven't tried any benchmarks.  Have any tests I can run on two kernels?
I'm currently running my machine with the patches and I haven't noticed
a difference.  Although I'm not doing database work, I'm still compiling
kernels.

Reason I'm asking, is that I wonder if the whole VM idea is a waste, or is
it only a problem on certain archs?

Perhaps move the whole of percpu_boot_alloc into the arch, and let it do
the allocation as is.  Could perhaps use some arch specific register to
calculate the entries.

OK, now I'm just rambling. I don't know,  have any other ideas on making
this more robust?  Or is this all in vain, and I should spend my evenings
walking around this beautiful town of Karlsruhe ;)

-- Steve

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