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 12:19:39 +0800

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]>

Reviewed-by: Wanpeng Li <[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)) {

s/node_spanned_pages/node_present_pages 

>+			deactivate_slab(s, page, c->freelist);
>+			c->page = NULL;
>+			c->freelist = NULL;
>+			goto new_slab;
>+		}
> 	}
>
> 	/*
>
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