Re: [RFC] slub: Simplify boot kmem_cache_cpu allocations

[Christoph Lameter <cl@linux-foundation.org>]

On Wed, 16 Jun 2010, Tejun Heo wrote:

> Hello,
>
> On 06/16/2010 06:33 PM, Christoph Lameter wrote:
> > On Wed, 16 Jun 2010, Tejun Heo wrote:
> >>> Tejun: Is it somehow possible to reliably use the alloc_percpu() on all
> >>> platforms during early boot before the slab allocator is up?
> >>
> >> Hmmm... first chunk allocation is done using bootmem, so if we give it
> >> enough to room (for both chunk itself and alloc map) so that it can
> >> serve till slab comes up, it should work fine.  I think what's
> >> important here is making up our minds and decide on how to order them.
> >> If the order is well defined, things can be made to work one way or
> >> the other.  What happened to the get-rid-of-bootmem effort?  Wouldn't
> >> that also interact with this?
> >
> > Ok how do we make sure that the first chunk has enough room?
>
> It's primarily controlled by PERCPU_DYNAMIC_RESERVE.  I don't think
> there will be any systematic way to do it other than sizing it
> sufficiently.  Can you calculate the upper bound?  The constant has
> been used primarily for optimization so how it's used needs to be
> audited if we wanna guarantee free space in the first chunk but I
> don't think it would be too difficult.

The upper bound is SLUB_PAGE_SHIFT * sizeof(struct kmem_cache_cpu).

Thats usually 14 * 104 bytes = 1456 bytes. This may increase to more
than 8k given the future plans to add queues into kmem_cache_cpu.



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