Re: [PATCH 3/5] mm: slub allocation fairness

Peter Zijlstra
2007-05-14T16:15:02Z

On Mon, 2007-05-14 at 08:49 -0700, Christoph Lameter wrote:
> On Mon, 14 May 2007, Peter Zijlstra wrote:
>
> > Index: linux-2.6-git/include/linux/slub_def.h
> > ===================================================================
> > --- linux-2.6-git.orig/include/linux/slub_def.h
> > +++ linux-2.6-git/include/linux/slub_def.h
> > @@ -52,6 +52,7 @@ struct kmem_cache {
> > struct kmem_cache_node *node[MAX_NUMNODES];
> > #endif
> > struct page *cpu_slab[NR_CPUS];
> > + int rank;
> > };
>
> Ranks as part of the kmem_cache structure? I thought this is a temporary
> thing?

No it needs to store the current state to verity subsequent allocations
their gfp flags against.

> > * Lock order:
> > @@ -961,6 +962,8 @@ static struct page *allocate_slab(struct
> > if (!page)
> > return NULL;
> >
> > + s->rank = page->index;
> > +
>
> Argh.... Setting a cache structure field from a page struct field? What
> about concurrency?

Oh, right; allocate_slab is not serialized itself.

> > @@ -1371,8 +1376,12 @@ static void *__slab_alloc(struct kmem_ca
> > gfp_t gfpflags, int node, void *addr, struct page *page)
> > {
> > void **object;
> > - int cpu = smp_processor_id();
> > + int cpu;
> > +
> > + if (page == FORCE_PAGE)
> > + goto force_new;
> >
> > + cpu = smp_processor_id();
> > if (!page)
> > goto new_slab;
> >
> > @@ -1405,6 +1414,7 @@ have_slab:
> > goto load_freelist;
> > }
> >
> > +force_new:
> > page = new_slab(s, gfpflags, node);
> > if (page) {
> > cpu = smp_processor_id();
> > @@ -1465,15 +1475,22 @@ static void __always_inline *slab_alloc(
> > struct page *page;
> > void **object;
> > unsigned long flags;
> > + int rank = slab_alloc_rank(gfpflags);
> >
> > local_irq_save(flags);
> > + if (slab_insufficient_rank(s, rank)) {
> > + page = FORCE_PAGE;
> > + goto force_alloc;
> > + }
> > +
> > page = s->cpu_slab[smp_processor_id()];
> > if (unlikely(!page || !page->lockless_freelist ||
> > - (node != -1 && page_to_nid(page) != node)))
> > + (node != -1 && page_to_nid(page) != node))) {
> >
> > +force_alloc:
> > object = __slab_alloc(s, gfpflags, node, addr, page);
> >
> > - else {
> > + } else {
> > object = page->lockless_freelist;
> > page->lockless_freelist = object[page->offset];
> > }
>
> This is the hot path. No modifications please.

Yes it is, but sorry, I have to. I really need to validate each slab
alloc its GFP flags. Thats what the whole thing is about, I thought you
understood that.