Re: [PATCH] kmalloc_percpu

[Rusty Russell]

In message <[e-mail]> you write:
> Rusty Russell <[e-mail]> wrote:
> >
> > It's a tradeoff, but I think it's worth it for a kmalloc_percpu which
> > is fast, space-efficient and numa-aware, since the code is needed
> > anyway.
> 
> I don't beleive that kmalloc_percpu() itself needs to be fast, as you say.
> 
> The code is _not_ NUMA-aware.  Is it?

No, it's arch-overridable.  You know me, I don't mess with
arch-specific stuff. 8)

> On 64GB 32-way that's 128MB of lowmem.  Unpopular.  I'd settle for a __setup
> thingy here, and a printk when the memory runs out.

Currently __setup is run far too late.  Fortunately Bartlomiej has
been working on moving __setup earlier, for other reasons (two-stage
parameter parsing).  In this case, it'd need to be before arch setup,
hmm...

> btw, what's wrong with leaving kmalloc_percpu() as-is, and only using this
> allocator for DEFINE_PERCPU()?

I figured that since the allocator is going to be there anyway, it
made sense to express kmalloc_percpu() in those terms.  If you think
the price is too high, I can respect that.

The more people use the __per_cpu_offset instead of
smp_processor_id(), the cheaper it gets, even to the stage where some
archs might want to express smp_processor_id() in terms of
__per_cpu_offset, rather than vice-versa.  Given that it's also more
space efficient (each var doesn't take one cacheline) too, I'd
recommend __alloc_percpu() of eg. single ints for long-lived objects
where I wouldn't recommend the current kmalloc_percpu().

Paul Mackerras points out that we could get the numa-aware allocation
plus "one big alloc" properties by playing with page mappings: reserve
1MB of virtual address, and map more pages as required.  I didn't
think that we'd need that yet, though.

So, numerous options, and you're smarter than me, so you can decide 8)
Rusty.
--
  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.