Re: [PATCH] slub: Don't throw away partial remote slabs if there is no local memory

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

On Tue, Jan 07, 2014 at 06:31:56PM +0900, Joonsoo Kim wrote:
>On Tue, Jan 07, 2014 at 05:21:45PM +0800, Wanpeng Li wrote:
>> On Tue, Jan 07, 2014 at 06:10:16PM +0900, Joonsoo Kim wrote:
>> >On Tue, Jan 07, 2014 at 04:48:40PM +0800, Wanpeng Li wrote:
>> >> Hi Joonsoo,
>> >> On Tue, Jan 07, 2014 at 04:41:36PM +0900, Joonsoo Kim wrote:
>> >> >On Tue, Jan 07, 2014 at 01:21:00PM +1100, Anton Blanchard wrote:
>> >> >> 
>> >> [...]
>> >> >Hello,
>> >> >
>> >> >I think that we need more efforts to solve unbalanced node problem.
>> >> >
>> >> >With this patch, even if node of current cpu slab is not favorable to
>> >> >unbalanced node, allocation would proceed and we would get the unintended memory.
>> >> >
>> >> 
>> >> We have a machine:
>> >> 
>> >> [    0.000000] Node 0 Memory:
>> >> [    0.000000] Node 4 Memory: 0x0-0x10000000 0x20000000-0x60000000 0x80000000-0xc0000000
>> >> [    0.000000] Node 6 Memory: 0x10000000-0x20000000 0x60000000-0x80000000
>> >> [    0.000000] Node 10 Memory: 0xc0000000-0x180000000
>> >> 
>> >> [    0.041486] Node 0 CPUs: 0-19
>> >> [    0.041490] Node 4 CPUs:
>> >> [    0.041492] Node 6 CPUs:
>> >> [    0.041495] Node 10 CPUs:
>> >> 
>> >> The pages of current cpu slab should be allocated from fallback zones/nodes 
>> >> of the memoryless node in buddy system, how can not favorable happen? 
>> >
>> >Hi, Wanpeng.
>> >
>> >IIRC, if we call kmem_cache_alloc_node() with certain node #, we try to
>> >allocate the page in fallback zones/node of that node #. So fallback list isn't
>> >related to fallback one of memoryless node #. Am I wrong?
>> >
>> 
>> Anton add node_spanned_pages(node) check, so current cpu slab mentioned
>> above is against memoryless node. If I miss something?
>
>I thought following scenario.
>
>memoryless node # : 1
>1's fallback node # : 0
>
>On node 1's cpu,
>
>1. kmem_cache_alloc_node (node 2)
>2. allocate the page on node 2 for the slab, now cpu slab is that one.
>3. kmem_cache_alloc_node (local node, that is, node 1)
>4. It check node_spanned_pages() and find it is memoryless node.
>So return node 2's memory.
>
>Is it impossible scenario?
>

Indeed, it can happen. 

Regards,
Wanpeng Li 

>Thanks.