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

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Message-ID: <Z-6T234S88wkH0uh@harry>
Date: Thu, 3 Apr 2025 22:57:47 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Oscar Salvador <osalvador@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        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 Thu, Apr 03, 2025 at 03:08:18PM +0200, David Hildenbrand wrote:
> On 03.04.25 15:02, David Hildenbrand wrote:
> > 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?
> 
> BTW, isn't description of slab_nodes wrong?
> 
> "Tracks for which NUMA nodes we have kmem_cache_nodes allocated." -- but as
> there is no freeing done in slab_mem_offline_callback(), isn't it always
> kept allocated?

It was, but not anymore :)

I think this patch series [1] forgot the fact that it changed the meaning
from 'NUMA nodes that have kmem_cache_node', to 'NUMA nodes that have normal
memory (that can be allocated as slab memory)'?

[1] https://lore.kernel.org/all/20210113131634.3671-1-vbabka@suse.cz

> 
> (probably I am missing something)
> 
> -- 
> Cheers,
> 
> David / dhildenb
> 
> 

-- 
Cheers,
Harry (formerly known as Hyeonggon)