linux-kernel - Re: [PATCHSET x86/core/percpu] improve the first percpu chunk allocation

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Date:	Tue, 24 Feb 2009 13:40:42 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Tejun Heo <tj@...nel.org>
Cc:	rusty@...tcorp.com.au, tglx@...utronix.de, x86@...nel.org,
	linux-kernel@...r.kernel.org, hpa@...or.com, jeremy@...p.org,
	cpw@....com, nickpiggin@...oo.com.au, ink@...assic.park.msu.ru
Subject: Re: [PATCHSET x86/core/percpu] improve the first percpu chunk
	allocation

* Tejun Heo <tj@...nel.org> wrote:

> Hello, Ingo.
> 
> Ingo Molnar wrote:
> > Hm, i think there still must be some basic misunderstanding 
> > somewhere here. Let me describe the design i described in the 
> > previous mail in more detail.
> > 
> > In one of your changelogs you state:
> > 
> > |    On NUMA, embedding allocator can't be used as different 
> > |    units can't be made to fall in the correct NUMA nodes.
> > 
> > This is a direct consequence of the unit/chunk abstraction,
> 
> Not at all.  That's an optimization for !NUMA.  The remap 
> allocator is what can be done on NUMA.  Chunking or not 
> doesn't make any difference in this regard.  The only 
> difference between chunking and not chunking is whether 
> separately allocated percpu offsets have more or less holes 
> inbetween them, which is irrelevant for all purposes.

It's not an optimization, it's a pessimisation :)

Please read what i wrote to you. We want the percpu static and 
dynamic areas to be _one and the same thing_. (With just the 
different that static allocations have a handy compile-time 
offset shortcut - but the access is still the same.)

Right now, with your latest code we still have this:

   * Use this to get to a cpu's version of the per-cpu object
   * dynamically allocated. Non-atomic access to the current  CPU's
   * version should probably be combined with get_cpu()/put_cpu().
   */
  #define per_cpu_ptr(ptr, cpu)   SHIFT_PERCPU_PTR((ptr), per_cpu_offset((cpu)))

This slows down per_cpu_ptr() and makes the dynamic percpu case 
a second-class citizen because most actual usages are for the 
current CPU, still have to go via the per_cpu_offset() 
indirection.

I.e. we have things like:

        const int cpu = get_cpu();

        u8 *scratch = *per_cpu_ptr(ipcomp_scratches, cpu);

Instead of a straight:

        u8 *scratch = *this_cpu_ptr(ipcomp_scratches);

We cannot do that optimization due to the NUMA and SMP 
assymetry. If NUMA and SMP had the same linear structure, as i 
suggested we do, we could do it.

Currently you rely on per_cpu_offset() indirection basically as 
a soft-TLB entry covering all dynamic allocations. That sucks.

Ok?

	Ingo
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