Re: [PATCH v3 net] net: skbuff: don't copy alloc_cpu in skb header copies

[Kuniyuki Iwashima <kuniyu@google.com>]

On Mon, 19 Oct 2026 20:41:52 +0300, Carolina Jubran wrote:
> Commit 68822bdf76f1 ("net: generalize skb freeing deferral to
> per-cpu lists") added alloc_cpu to struct sk_buff and placed it in the
> headers group copied by __copy_skb_header().
> 
> This is wrong for newly allocated skbs. For example, skb_segment()
> allocates a new skb and records the current CPU in alloc_cpu, but
> __copy_skb_header() overwrites it with the value from the original skb.
> 
> If the original skb was allocated on another CPU, the segment may later
> be treated as remotely allocated and unnecessarily deferred there when
> it is freed.
> 
> Move alloc_cpu out of the headers group so it is preserved across
> header copies. Copy it explicitly in skb_clone(), which allocates
> the skb object directly instead of going through __alloc_skb().
> 
> This was discovered while investigating a throughput regression with
> software IPsec. The load used 512 IPsec tunnels carrying traffic over
> VXLAN, with 512 parallel iperf3 flows and an MTU of 4500.
> 
> Before the fix:
> 
>   16.09%  queued_spin_lock_slowpath
>              |
>              --15.90%--_raw_spin_lock_irqsave
>                        |
>                        --9.37%--__slab_free
>                                  kfree
>                                  skb_release_data
>                                  napi_consume_skb
>                                  mlx5e_poll_tx_cq
> 
> After the fix:
> 
>    6.84%  queued_spin_lock_slowpath
>              |
>              --6.57%--_raw_spin_lock_irqsave
>                       |
>                       --5.00%--__slab_free
>                                 kfree
>                                 skb_release_data
>                                 napi_consume_skb
>                                 mlx5e_poll_tx_cq
> 
> Fixes: 68822bdf76f1 ("net: generalize skb freeing deferral to per-cpu lists")
> Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
> Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
> Reviewed-by: Björn Töpel <bjorn@kernel.org>
> Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>