Subject: Re: hackbench regression due to commit 9dfc6e68bfe6e

From: Pekka Enberg <[email-masked]>
Date: Wed, 07 Apr 2010 19:52:56 +0300

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...