Re: [PATCH 0/5] make slab gfp fair

Peter Zijlstra
2007-05-14T20:03:47Z

On Mon, 2007-05-14 at 12:56 -0700, Christoph Lameter wrote:
> On Mon, 14 May 2007, Peter Zijlstra wrote:
>
> > > You can pull the big switch (only on a SLUB slab I fear) to switch
> > > off the fast path. Do SetSlabDebug() when allocating a precious
> > > allocation that should not be gobbled up by lower level processes.
> > > Then you can do whatever you want in the __slab_alloc debug section and we
> > > wont care because its not the hot path.
> >
> > One allocator is all I need; it would just be grand if all could be
> > supported.
> >
> > So what you suggest is not placing the 'emergency' slab into the regular
> > place so that normal allocations will not be able to find it. Then if an
> > emergency allocation cannot be satified by the regular path, we fall
> > back to the slow path and find the emergency slab.
>
> Hmmm.. Maybe we could do that.... But what I had in mind was simply to
> set a page flag (DebugSlab()) if you know in alloc_slab that the slab
> should be only used for emergency allocation. If DebugSlab is set then the
> fastpath will not be called. You can trap all allocation attempts and
> insert whatever fancy logic you want in the debug path since its not
> performance critical.

I might have missed some detail when I looked at SLUB, but I did not see
how setting SlabDebug would trap subsequent allocations to that slab.

> > The thing is; I'm not needing any speed, as long as the machine stay
> > alive I'm good. However others are planing to build a full reserve based
> > allocator to properly fix the places that now use __GFP_NOFAIL and
> > situation such as in add_to_swap().
>
> Well I have version of SLUB here that allows you do redirect the alloc
> calls at will. Adds a kmem_cache_ops structure and in the kmem_cache_ops
> structure you can redirect allocation and freeing of slabs (not objects!)
> at will. Would that help?

I'm not sure; I need kmalloc as well.