Re: [PATCH 0/2] Implement numa node notifier

David Hildenbrand
2025-04-03T13:02:39Z

On 02.04.25 19:03, Oscar Salvador wrote:
> On Wed, Apr 02, 2025 at 06:06:51PM +0200, Vlastimil Babka wrote:
>> What if we had two chains:
>>
>> register_node_notifier()
>> register_node_normal_notifier()
>>
>> I think they could have shared the state #defines and struct node_notify
>> would have just one nid and be always >= 0.
>>
>> Or would it add too much extra boilerplate and only slab cares?
>
> We could indeed go on that direction to try to decouple
> status_change_nid from status_change_nid_normal.
>
> Although as you said, slub is the only user of status_change_nid_normal
> for the time beign, so I am not sure of adding a second chain for only
> one user.
>
> Might look cleaner though, and the advantatge is that slub would not get
> notified for nodes adquiring only ZONE_MOVABLE.
>
> Let us see what David thinks about it.

I'd hope we'd be able to get rid of the _normal stuff completely, it's seems
way to specialized.

We added that in

commit b9d5ab2562eceeada5e4837a621b6260574dd11d
Author: Lai Jiangshan <laijs@cn.fujitsu.com>
Date: Tue Dec 11 16:01:05 2012 -0800

slub, hotplug: ignore unrelated node's hot-adding and hot-removing

SLUB only focuses on the nodes which have normal memory and it ignores the
other node's hot-adding and hot-removing.

Aka: if some memory of a node which has no onlined memory is online, but
this new memory onlined is not normal memory (for example, highmem), we
should not allocate kmem_cache_node for SLUB.

And if the last normal memory is offlined, but the node still has memory,
we should remove kmem_cache_node for that node. (The current code delays
it when all of the memory is offlined)

So we only do something when marg->status_change_nid_normal > 0.
marg->status_change_nid is not suitable here.

The same problem doesn't exist in SLAB, because SLAB allocates kmem_list3
for every node even the node don't have normal memory, SLAB tolerates
kmem_list3 on alien nodes. SLUB only focuses on the nodes which have
normal memory, it don't tolerate alien kmem_cache_node. The patch makes
SLUB become self-compatible and avoids WARNs and BUGs in rare conditions.

How "bad" would it be if we do the slab_mem_going_online_callback() etc even
for completely-movable nodes? I assume one kmem_cache_alloc() per slab_caches.

slab_mem_going_offline_callback() only does shrinking, #dontcare

Looking at slab_mem_offline_callback(), we never even free the caches either
way when offlining. So the implication would be that we would have movable-only nodes
set in slab_nodes.

We don't expect many such nodes, so ... do we care?

--
Cheers,

David / dhildenb