Re: [RFC] Scalable statistics counters using kmalloc_percpu

[Andrew Morton <akpm@zip.com.au>]

Rusty Russell wrote:
> 
> In message <3D422553.6B126242@zip.com.au> you write:
> > Good.  And will it be possible to iterate across all CPUs
> > without having to iterate across NR_CPUS?
> 
> Hmm, define *all cpus* please?  All online cpus?  All possible CPUs?

All possible.

> Interface discussed with DaveM was: first_cpu(), next_cpu(cpu), which
> covers online CPUs, and gives a nice interface for things like irq
> balancers which want to snarf the next online cpu.
> 
> Like it?

	for (cpu = first_cpu(); cpu != (what?); cpu = next_cpu(cpu))

that'll work.
 
> ...
> 
> > The is pretty much entirely wasted memory, and it will only get
> > worse. Making NR_CPUS compile-time configurable is a lame solution.
> > Wasting the memory is out of the question.
> >
> > Dynamic allocation is the only thing left, yes?
> 
> Um, no!  Here is the plan:
> 
> 1) change per-cpu data to only allocate data for cpus where
>    cpu_possible(i) (add cache coloring and NUMA allocation as desired).
> 
> 2) Convert non-modular cases to use per-cpu data (once the interface
>    changes again, <SIGH>).
> 
> We'll end up using *less* memory than before.  We're just doing it in
> easy stages.
> 
> Feel better now?

Yup.

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