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

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Message-ID: <e1ebfafa-f063-4340-b577-d1b6b2fb5d11@redhat.com>
Date: Thu, 3 Apr 2025 15:02:25 +0200
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...e.de>, Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Hyeonggon Yoo <42.hyeyoo@...il.com>,
 mkoutny@...e.com, Dan Williams <dan.j.williams@...el.com>,
 Jonathan Cameron <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH 0/2] Implement numa node notifier

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@...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