Re: hackbench regression due to commit 9dfc6e68bfe6e

[Pekka Enberg]

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?

Oh, sorry, I think it's actually '____cacheline_aligned_in_smp' (with 
four underscores) for per-cpu data. Confusing...
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