Re: [PATCH] per-cpu change

[David Mosberger <davidm@napali.hpl.hp.com>]

>>>>> On Fri, 02 May 2003 18:28:34 +1000, Rusty Russell <rusty@rustcorp.com.au> said:

  Rusty> Hi David, To implement kmalloc_percpu to use the same
  Rusty> primitives that normal (static) per-cpu vars use, I have made
  Rusty> some changes.  This is required for sane per-cpu variable
  Rusty> support in modules, but the implementation also makes
  Rusty> allocation v. efficient.  I've had to make more assumptions
  Rusty> about the per-cpu layout though.

  Rusty> 	IA64 is the only arch which doesn't use the generic
  Rusty> struct (x86-64 seems to be partway 8), so I wanted to check
  Rusty> with you.  Basically, we over-allocate up front, and pass out
  Rusty> the rest through the __alloc_percpu interface.  I've made the
  Rusty> setup_per_cpu_areas() idempotent, so you should simply be
  Rusty> able to call that from your cpu_init() (the later generic
  Rusty> call will do nothing).

  Rusty> 	Please tell me if this is too restrictive for IA64.
  Rusty> Ideally it should be a simplification: you just need to
  Rusty> override __get_cpu_var(var) and __get_cpu_ptr(var) for SMP in
  Rusty> asm/percpu.h.  You might want to set PERCPU_POOL_SIZE (16k
  Rusty> would be plenty for my machine, but you currently do 64k).

We pin the per-CPU area into the TLB, so the size of this area is
limited to one of the supported page sizes (yes, we _could_ pin
multiple TLB entries, but I do not think that would be economical).
At the moment, we use a 64KB page size.  So as long as the per-CPU
area is guaranteed never to be bigger than that, we should be fine.

Two other comments:

 - On NUMA, you want to allocate the per-CPU areas in node-local memory.
 - It's important to initialize the per-CPU area early enough.  We
   currently do it in cpu_init().  Anything later would probably break
   things.

I am a little concerned with supporting per-CPU storage in modules.
What happens if the per-CPU area fills up?  With the normal kernel,
you'll get a link-time error, but with modules, you won't know until
you try to insert a module.  But the alternative of using something
similar to the user-level TLS model doesn't look exactly attractive
either.

	--david