netdev - Re: [PATCH v6 net-next 13/13] mlx5: support BIG TCP packets

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Date:   Thu, 12 May 2022 22:49:45 -0700
From:   Saeed Mahameed <saeedm@...dia.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Alexander Duyck <alexanderduyck@...com>,
        Coco Li <lixiaoyan@...gle.com>,
        Tariq Toukan <tariqt@...dia.com>,
        Leon Romanovsky <leon@...nel.org>
Subject: Re: [PATCH v6 net-next 13/13] mlx5: support BIG TCP packets

On 12 May 21:34, Eric Dumazet wrote:
>On Thu, May 12, 2022 at 9:29 PM Saeed Mahameed <saeedm@...dia.com> wrote:
>>
>> On 12 May 11:02, Paolo Abeni wrote:
>> >On Thu, 2022-05-12 at 01:40 -0700, Saeed Mahameed wrote:
>> >> On 09 May 20:32, Eric Dumazet wrote:
>> >> > From: Coco Li <lixiaoyan@...gle.com>
>> >> >
>> >> > mlx5 supports LSOv2.
>> >> >
>> >> > IPv6 gro/tcp stacks insert a temporary Hop-by-Hop header
>> >> > with JUMBO TLV for big packets.
>> >> >
>> >> > We need to ignore/skip this HBH header when populating TX descriptor.
>> >> >
>> >>
>> >> Sorry i didn't go through all the documentations or previous discussions,
>> >> please bare with me, so why not clear HBH just before calling the
>> >> driver xmit ndo ?
>> >
>> >I guess this way is more efficient: the driver copies IP hdr and TCP
>> >hdr directly in the correct/final location into the tx descriptor,
>> >otherwise the caller would have to memmove L2/L3 just before the driver
>> >copies them again.
>> >>
>>
>> memmove(sizeof(L2/L3)) is not that bad when done only every 64KB+.
>> it's going to be hard to repeat this and maintain this across all drivers
>> only to get this micro optimization that I doubt it will be even measurable.
>
>We prefer not changing skb->head, this would break tcpdump.
>

in that case we can provide a helper to the drivers to call, just before
they start processing the skb.

>Surely calling skb_cow_head() would incur a cost.
>

Sure, but the benefit of this patch outweighs this cost by orders of
magnitude, you pay an extra 0.1$ for a cleaner code, and you still
get your 64K$ BIG TCP cash. 

>As I suggested, we can respin the series without the mlx5 patch, this
>is totally fine for us, if we can avoid missing 5.19 train.

To be clear, I am not nacking, Tariq already reviewed and gave his blessing,
and i won't resist this patch on v6. I am Just suggesting an improvement
to code readability and scalability to other drivers.

FWIW:
Acked-by: Saeed Mahameed <saeedm@...dia.com>