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SubjectRe: hackbench regression due to commit 9dfc6e68bfe6e
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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