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From: =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>
To: Carolina Jubran <cjubran@nvidia.com>, "David S . Miller"
 <davem@davemloft.net>, Eric Dumazet <edumazet@kernel.org>, Jakub Kicinski
 <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>, Jason Xing <kerneljasonxing@gmail.com>,
 Kuniyuki Iwashima <kuniyu@google.com>, Jiayuan Chen
 <jiayuan.chen@linux.dev>, netdev@vger.kernel.org,
 linux-kernel@vger.kernel.org, Carolina Jubran <cjubran@nvidia.com>, Dragos
 Tatulea <dtatulea@nvidia.com>
Subject: Re: [PATCH net] net: skbuff: don't copy alloc_cpu in skb header copies
In-Reply-To: <20261012124544.2018007-1-cjubran@nvidia.com>
References: <20261012124544.2018007-1-cjubran@nvidia.com>
Date: Mon, 12 Oct 2026 17:26:39 +0200
Message-ID: <87mrsjaqj4.fsf@all.your.base.are.belong.to.us>
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Carolina Jubran <cjubran@nvidia.com> writes:

> 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. Initialize 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 lis=
ts")
> Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
> Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>

Reviewed-by: Bj=C3=B6rn T=C3=B6pel <bjorn@kernel.org>