Re: [PATCH] tree/tiny rcu: Add debug RCU head option (v2)

[Paul E. McKenney]

On Fri, Mar 19, 2010 at 07:34:07AM -0500, Christoph Lameter wrote:
> On Fri, 19 Mar 2010, Lai Jiangshan wrote:
> 
> > but SLUB opposed me, so I did not post it.
> >
> > @ slub.c
> > static void free_slab(struct kmem_cache *s, struct page *page)
> > {
> > 	if (unlikely(s->flags & SLAB_DESTROY_BY_RCU)) {
> > 		/*
> > 		 * RCU free overloads the RCU head over the LRU
> > 		 */
> > 		struct rcu_head *head = (void *)&page->lru;
> >
> > 		call_rcu(head, rcu_free_slab);
> > 	} else
> > 		__free_slab(s, page);
> > }
> >
> > I think it is not your patch's fault, but the above code has
> > already existed.
> >
> > In your patch, struct rcu_head::debug will share the same memory of
> > the other field of struct page, (especially share the struct page::flags
> > which I encountered)
> 
> Right. I like the smallness of the RCU structure but if you must
> increase its size then we could:
> 
> A) put the rcu structure at the end of each page like SLOB
>    but that will take away memory that could be used by objects.
> 
> B) Move the rcu object up by a word so that it overloads the index field
>    in the page struct. That would give another exact fit but will cause
>    trouble again if you bloat the structure some more.

The size increase is strictly under a debug option -- the non-debug
size remains the same as always.  And I expect that in the longer term,
we will be able to shrink the size back down even with this debug option
available.

One question: do any of the other allocators care about the size of the
struct rcu_head?  If not, a temporary work-around would be to simply
prohibit use of this debug option when running with SLUB.

							Thanx, Paul
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