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

From: Wanpeng Li <[email-masked]>
Date: Tue, 7 Jan 2014 17:52:31 +0800

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:
>> 
>> We noticed a huge amount of slab memory consumed on a large ppc64 box:
>> 
>> Slab:            2094336 kB
>> 
>> Almost 2GB. This box is not balanced and some nodes do not have local
>> memory, causing slub to be very inefficient in its slab usage.
>> 
>> Each time we call kmem_cache_alloc_node slub checks the per cpu slab,
>> sees it isn't node local, deactivates it and tries to allocate a new
>> slab. On empty nodes we will allocate a new remote slab and use the
>> first slot, but as explained above when we get called a second time
>> we will just deactivate that slab and retry.
>> 
>> As such we end up only using 1 entry in each slab:
>> 
>> slab                    mem  objects
>>                        used   active
>> ------------------------------------
>> kmalloc-16384       1404 MB    4.90%
>> task_struct          668 MB    2.90%
>> kmalloc-128          193 MB    3.61%
>> kmalloc-192          152 MB    5.23%
>> kmalloc-8192          72 MB   23.40%
>> kmalloc-16            64 MB    7.43%
>> kmalloc-512           33 MB   22.41%
>> 
>> The patch below checks that a node is not empty before deactivating a
>> slab and trying to allocate it again. With this patch applied we now
>> use about 352MB:
>> 
>> Slab:             360192 kB
>> 
>> And our efficiency is much better:
>> 
>> slab                    mem  objects
>>                        used   active
>> ------------------------------------
>> kmalloc-16384         92 MB   74.27%
>> task_struct           23 MB   83.46%
>> idr_layer_cache       18 MB  100.00%
>> pgtable-2^12          17 MB  100.00%
>> kmalloc-65536         15 MB  100.00%
>> inode_cache           14 MB  100.00%
>> kmalloc-256           14 MB   97.81%
>> kmalloc-8192          14 MB   85.71%
>> 
>> Signed-off-by: Anton Blanchard <[email-masked]>
>> ---
>> 
>> Thoughts? It seems like we could hit a similar situation if a machine
>> is balanced but we run out of memory on a single node.
>> 
>> Index: b/mm/slub.c
>> ===================================================================
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2278,10 +2278,17 @@ redo:
>>  
>>  	if (unlikely(!node_match(page, node))) {
>>  		stat(s, ALLOC_NODE_MISMATCH);
>> -		deactivate_slab(s, page, c->freelist);
>> -		c->page = NULL;
>> -		c->freelist = NULL;
>> -		goto new_slab;
>> +
>> +		/*
>> +		 * If the node contains no memory there is no point in trying
>> +		 * to allocate a new node local slab
>> +		 */
>> +		if (node_spanned_pages(node)) {
>> +			deactivate_slab(s, page, c->freelist);
>> +			c->page = NULL;
>> +			c->freelist = NULL;
>> +			goto new_slab;
>> +		}
>>  	}
>>  
>>  	/*
>
>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.
>
>And there is one more problem. Even if we have some partial slabs on
>compatible node, we would allocate new slab, because get_partial() cannot handle
>this unbalance node case.
>
>To fix this correctly, how about following patch?
>
>Thanks.
>
>------------->8--------------------
>diff --git a/mm/slub.c b/mm/slub.c
>index c3eb3d3..a1f6dfa 100644
>--- a/mm/slub.c
>+++ b/mm/slub.c
>@@ -1672,7 +1672,19 @@ static void *get_partial(struct kmem_cache *s, gfp_t flags, int node,
> {
>        void *object;
>        int searchnode = (node == NUMA_NO_NODE) ? numa_node_id() : node;
>+       struct zonelist *zonelist;
>+       struct zoneref *z;
>+       struct zone *zone;
>+       enum zone_type high_zoneidx = gfp_zone(flags);
>
>+       if (!node_present_pages(searchnode)) {
>+               zonelist = node_zonelist(searchnode, flags);
>+               for_each_zone_zonelist(zone, z, zonelist, high_zoneidx) {
>+                       searchnode = zone_to_nid(zone);
>+                       if (node_present_pages(searchnode))
>+                               break;
>+               }
>+       }

Why change searchnode instead of depending on fallback zones/nodes in 
get_any_partial() to allocate partial slabs?

Regards,
Wanpeng Li 

>        object = get_partial_node(s, get_node(s, searchnode), c, flags);
>        if (object || node != NUMA_NO_NODE)
>                return object;
>
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