Re: [PATCH v2] net: iterate online nodes in skb_defer_free_flush()

[Jakub Kicinski]

On Fri, 11 Sep 2026 10:38:04 +0800 Kris Pan wrote:
> skb_attempt_defer_free() only queues skbs on the current CPU's node
> (numa_node_id() of a running CPU), which is always online, so the
> flush loop never needs to visit nodes that are merely possible.
> 
> for_each_node() walks node_possible_map.  On machines where the
> possible map is much larger than the online map -- e.g. a POWER10
> LPAR with 32 possible but 1 online node -- the flush loop touches 31
> cold, always-empty per-node lists on every softirq pass, showing up
> as skb_defer_free_flush() and _find_next_bit() overhead.
> 
> Use for_each_online_node() to iterate only node_online_map.
> 
> Loopback UDP throughput in a QEMU guest with 32 possible / 1 online
> nodes (bench_udp, 8 senders, 6 interleaved runs) improves by ~5%.

Clashiko confirms this is racy:

https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260911023804.3503989-1-kris.pan@xxxxxxxxx
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