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

Nishanth Aravamudan
2014-01-25T00:17:08Z

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