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SubjectRe: hackbench regression due to commit 9dfc6e68bfe6e
On Wed, 7 Apr 2010, Pekka Enberg wrote:

> Christoph Lameter wrote:
> > I wonder if this is not related to the kmem_cache_cpu structure straggling
> > cache line boundaries under some conditions. On 2.6.33 the kmem_cache_cpu
> > structure was larger and therefore tight packing resulted in different
> > alignment.
> >
> > Could you see how the following patch affects the results. It attempts to
> > increase the size of kmem_cache_cpu to a power of 2 bytes. There is also
> > the potential that other per cpu fetches to neighboring objects affect the
> > situation. We could cacheline align the whole thing.
> >
> > ---
> > include/linux/slub_def.h | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> > Index: linux-2.6/include/linux/slub_def.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/slub_def.h 2010-04-07 11:33:50.000000000
> > -0500
> > +++ linux-2.6/include/linux/slub_def.h 2010-04-07 11:35:18.000000000
> > -0500
> > @@ -38,6 +38,11 @@ struct kmem_cache_cpu {
> > void **freelist; /* Pointer to first free per cpu object */
> > struct page *page; /* The slab from which we are allocating */
> > int node; /* The node of the page (or -1 for debug) */
> > +#ifndef CONFIG_64BIT
> > + int dummy1;
> > +#endif
> > + unsigned long dummy2;
> > +
> > #ifdef CONFIG_SLUB_STATS
> > unsigned stat[NR_SLUB_STAT_ITEMS];
> > #endif
>
> Would __cacheline_aligned_in_smp do the trick here?

This is allocated via the percpu allocator. We could specify cacheline
alignment there but that would reduce the density. You basically need 4
words for a kmem_cache_cpu structure. A number of those fit into one 64
byte cacheline.



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