Re: [PATCH v2] mm/mempolicy: recompute automatic interleave weights on memory hotplug

[David Hildenbrand (Arm)]

On 10/30/26 17:12, Gregory Price (Meta) wrote:
> In auto mode, weighted interleave derives each node's weight from its
> bandwidth, reduced over the nodes in N_MEMORY. HMAT and CXL report a
> node's bandwidth once, when the node first gains memory, and nothing
> else on the hotplug path recomputes the weights.
> 
> This can result in a major skew in distribution and overload tiers
> with more data than their respective bandwidth can support, resulting
> in a performance regression.
> 
> With HMAT bandwidths of 100G for nodes 0-3, and 50G and 25G for kmem
> nodes 4 and 5, weighted_interleave/nodeN reads:
> 
>   event             node 0-3   node 4   node 5
>   node 4 online         7         3        -
>   node 4 removed        7         -        -
>   node 5 online         7         -        1
>   node 4 back           7         1        1
>   Expected:             6         3        1
> 
> Toggling auto off and on generates the correct result, but we should
> detect this and fix it during hotplug. Recompute the weights from the
> recorded bandwidth whenever a node gains/loses its first/last memory.
> 
> The notifier cannot fail the hotplug, which has already happened, so
> warn once if the new weights cannot be allocated. The next recompute
> starts from the full bandwidth table and corrects them.
> 
> mempolicy_set_node_perf() now only records the bandwidth and calls the
> same helper, so the bandwidth is kept even if allocating the new weights
> fails. wi_state_lock is dropped between the two steps, which should be
> safe because racing writer simply causes it to run with newer state.
> 
> Fixes: e341f9c3c841 ("mm/mempolicy: Weighted Interleave Auto-tuning")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
> ---

It's a good idea to spell out what changed (and why) to make review easier ;)

@Andrew I can pick this for the next merge window, or do you want to merge this
through your tree?


-- 
Cheers,

David