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

From: Nishanth Aravamudan <[email-masked]>
Date: Fri, 24 Jan 2014 16:16:43 -0800

On 24.01.2014 [15:49:33 -0800], David Rientjes wrote:
> On Fri, 24 Jan 2014, Nishanth Aravamudan wrote:
> 
> > > I think the problem is a memoryless node being used for kmalloc_node() so 
> > > we need to decide where to enforce node_present_pages().  __slab_alloc() 
> > > seems like the best candidate when !node_match().
> > 
> > Actually, this is effectively what Anton's patch does, except with
> > Wanpeng's adjustment to use node_present_pages(). Does that seem
> > sufficient to you?
> > 
> 
> I don't see that as being the effect of Anton's patch.  We need to use 
> numa_mem_id() as Christoph mentioned when a memoryless node is passed for 
> the best NUMA locality.  Something like this:

Thank you for clarifying and providing  a test patch. I ran with this on
the system showing the original problem, configured to have 15GB of
memory.

With your patch after boot:

MemTotal:       15604736 kB
MemFree:         8768192 kB
Slab:            3882560 kB
SReclaimable:     105408 kB
SUnreclaim:      3777152 kB

With Anton's patch after boot:

MemTotal:       15604736 kB
MemFree:        11195008 kB
Slab:            1427968 kB
SReclaimable:     109184 kB
SUnreclaim:      1318784 kB

I know that's fairly unscientific, but the numbers are reproducible. 

For what it's worth, a sample of the unmodified numbers:

MemTotal:       15317632 kB
MemFree:         5023424 kB
Slab:            7176064 kB
SReclaimable:     106816 kB
SUnreclaim:      7069248 kB

So it's an improvement, but something is still causing us to (it seems)
be pretty inefficient with the slabs.

> diff --git a/mm/slub.c b/mm/slub.c
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2278,10 +2278,14 @@ 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 (unlikely(!node_present_pages(node)))
> +			node = numa_mem_id();
> +		if (!node_match(page, node)) {
> +			deactivate_slab(s, page, c->freelist);
> +			c->page = NULL;
> +			c->freelist = NULL;
> +			goto new_slab;
> +		}

Semantically, and please correct me if I'm wrong, this patch is saying
if we have a memoryless node, we expect the page's locality to be that
of numa_mem_id(), and we still deactivate the slab if that isn't true.
Just wanting to make sure I understand the intent.

What I find odd is that there are only 2 nodes on this system, node 0
(empty) and node 1. So won't numa_mem_id() always be 1? And every page
should be coming from node 1 (thus node_match() should always be true?)

Thanks,
Nish