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SubjectRe: Driver has suspect GRO implementation, TCP performance may be compromised.
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Dear Eric,


Sorry for the late reply.


On 5/29/19 6:00 PM, Eric Dumazet wrote:
> On Wed, May 29, 2019 at 7:49 AM Paul Menzel <pmenzel@molgen.mpg.de> wrote:

>> On 05/28/19 19:18, Eric Dumazet wrote:
>>> On 5/28/19 8:42 AM, Paul Menzel wrote:
>>
>>>> Occasionally, Linux outputs the message below on the workstation Dell
>>>> OptiPlex 5040 MT.
>>>>
>>>> TCP: net00: Driver has suspect GRO implementation, TCP performance may be compromised.
>>>>
>>>> Linux 4.14.55 and Linux 5.2-rc2 show the message, and the WWW also
>>>> gives some hits [1][2].
>>>>
>>>> ```
>>>> $ sudo ethtool -i net00
>>>> driver: e1000e
>>>> version: 3.2.6-k
>>>> firmware-version: 0.8-4
>>>> expansion-rom-version:
>>>> bus-info: 0000:00:1f.6
>>>> supports-statistics: yes
>>>> supports-test: yes
>>>> supports-eeprom-access: yes
>>>> supports-register-dump: yes
>>>> supports-priv-flags: no
>>>> ```
>>>>
>>>> Can the driver e1000e be improved?
>>>>
>>>> Any idea, what triggers this, as I do not see it every boot? Download
>>>> of big files?
>>>>
>>> Maybe the driver/NIC can receive frames bigger than MTU, although this would be strange.
>>>
>>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>>> index c61edd023b352123e2a77465782e0d32689e96b0..cb0194f66125bcba427e6e7e3cacf0c93040ef61 100644
>>> --- a/net/ipv4/tcp_input.c
>>> +++ b/net/ipv4/tcp_input.c
>>> @@ -150,8 +150,10 @@ static void tcp_gro_dev_warn(struct sock *sk, const struct sk_buff *skb,
>>> rcu_read_lock();
>>> dev = dev_get_by_index_rcu(sock_net(sk), skb->skb_iif);
>>> if (!dev || len >= dev->mtu)
>>> - pr_warn("%s: Driver has suspect GRO implementation, TCP performance may be compromised.\n",
>>> - dev ? dev->name : "Unknown driver");
>>> + pr_warn("%s: Driver has suspect GRO implementation, TCP performance may be compromised."
>>> + " len %u mtu %u\n",
>>> + dev ? dev->name : "Unknown driver",
>>> + len, dev ? dev->mtu : 0);
>>> rcu_read_unlock();
>>> }
>>> }
>>
>> I applied your patch on commit 9fb67d643 (Merge tag 'pinctrl-v5.2-2' of
>> git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-pinctrl):
>>
>> [ 5507.291769] TCP: net00: Driver has suspect GRO implementation, TCP performance may be compromised. len 1856 mtu 1500
>
>
> The 'GRO' in the warning can be probably ignored, since this NIC does
> not implement its own GRO.
>
> You can confirm this with this debug patch:
>
> diff --git a/drivers/net/ethernet/intel/e1000e/netdev.c
> b/drivers/net/ethernet/intel/e1000e/netdev.c
> index 0e09bede42a2bd2c912366a68863a52a22def8ee..014a43ce77e09664bda0568dd118064b006acd67
> 100644
> --- a/drivers/net/ethernet/intel/e1000e/netdev.c
> +++ b/drivers/net/ethernet/intel/e1000e/netdev.c
> @@ -561,6 +561,9 @@ static void e1000_receive_skb(struct e1000_adapter *adapter,
> if (staterr & E1000_RXD_STAT_VP)
> __vlan_hwaccel_put_tag(skb, htons(ETH_P_8021Q), tag);
>
> + if (skb->len > netdev->mtu)
> + pr_err_ratelimited("received packet bigger (%u) than
> MTU (%u)\n",
> + skb->len, netdev->mtu);
> napi_gro_receive(&adapter->napi, skb);
> }

With this patch applied, I unfortunately could not trigger the condition
anymore. No idea why. Or is that expected?

(As a side note, plain Linux 5.2.2 still shows the warning.)


Kind regards,

Paul

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