Re: [RFC PATCH 2/2] mm/vmscan.c: Prevent allocating shrinker_info on offlined nodes

[Michal Hocko <mhocko@suse.com>]

On Mon 06-12-21 15:30:37, Vlastimil Babka wrote:
> On 12/6/21 15:21, Michal Hocko wrote:
> > On Mon 06-12-21 15:08:10, David Hildenbrand wrote:
> >> 
> >> >> But there might be more missing. Onlining a new zone will get more
> >> >> expensive in setups with a lot of possible nodes (x86-64 shouldn't
> >> >> really be an issue in that regard).
> >> > 
> >> > Honestly, I am not really concerned by platforms with too many nodes
> >> > without any memory. If they want to shoot their feet then that's their
> >> > choice. We can optimize for those if they ever prove to be standar.
> >> >  
> >> >> If we want stable backports, we'll want something simple upfront.
> >> > 
> >> > For stable backports I would be fine by doing your NODE_DATA check in
> >> > the allocator. In upstream I think we should be aiming for a more robust
> >> > solution that is also easier to maintain further down the line. Even if
> >> > that is an investment at this momemnt because the initialization code is
> >> > a mess.
> >> > 
> >> 
> >> Agreed. I would be curious *why* we decided to dynamically allocate the
> >> pgdat. is this just a historical coincidence or was there real reason to
> >> not allocate it for all possible nodes during boot?
> > 
> > I don't know but if I was to guess the most likely explanation would be
> > that the numa init code was in a similar order as now and it was easier
> > to simply allocate a pgdat when a new one was onlined.
> > 9af3c2dea3a3 ("[PATCH] pgdat allocation for new node add (call pgdat allocation)")
> > doesn't really tell much.
> 
> I don't know if that's true for pgdat specifically, but generally IMHO the
> advantages of allocating during/after online instead for each possible is
> - memory savings when some possible node is actually never online
> - at least in some cases, the allocations can be local to the node in
> question where the advantages is
>   - faster access
>   - less memory occupied on nodes that are earlier online, especially node 0
> 
> So while the approach of allocate on boot for all possible nodes instead of
> just online nodes has advantages of being generally safer and simpler (no
> memory hotplug callbacks etc), we should also be careful not to overdo this
> approach so we don't end up with Node 0 memory filled with structures used
> for nodes 1-X that are just onlined later. I imagine that could be a problem
> even for "sane" archs that don't have tons of possible, but offline nodes.

Yes this can indeed turn out to be a problem as the memory allocations
scales not only with numa nodes but memcgs as well. The later one being
a more visible one.

> Concretely, pgdat should probably be fine, but things like all shrinkers?
> Maybe less so.

Yeah, right. But for that purpose the concept of online_node is just
misleading. You would need a check whether the node is populated with
memory and implement hotplug notifiers.

-- 
Michal Hocko
SUSE Labs