netdev - Re: [PATCH net-next] net/mlx5e: Transmit small messages in linear skb

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Message-ID: <54945b8c-8328-4c34-982c-9a92ebab5b1c@linux.ibm.com>
Date: Wed, 11 Dec 2024 17:19:09 +0100
From: Alexandra Winter <wintera@...ux.ibm.com>
To: Dragos Tatulea <dtatulea@...dia.com>, Eric Dumazet <edumazet@...gle.com>
Cc: Rahul Rameshbabu <rrameshbabu@...dia.com>,
        Saeed Mahameed <saeedm@...dia.com>, Tariq Toukan <tariqt@...dia.com>,
        Leon Romanovsky <leon@...nel.org>, David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
        Andrew Lunn <andrew+netdev@...n.ch>,
        Nils Hoppmann <niho@...ux.ibm.com>, netdev@...r.kernel.org,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Thorsten Winkler <twinkler@...ux.ibm.com>,
        Simon Horman <horms@...nel.org>,
        Niklas Schnelle <schnelle@...ux.ibm.com>
Subject: Re: [PATCH net-next] net/mlx5e: Transmit small messages in linear skb



On 10.12.24 12:49, Dragos Tatulea wrote:
> 
> 
> On 06.12.24 16:25, Alexandra Winter wrote:
>>
>>
>> On 04.12.24 15:36, Eric Dumazet wrote:
>>> I would suggest the opposite : copy the headers (typically less than
>>> 128 bytes) on a piece of coherent memory.
>>>
>>> As a bonus, if skb->len is smaller than 256 bytes, copy the whole skb.
>>>
>>> include/net/tso.h and net/core/tso.c users do this.
>>>
>>> Sure, patch is going to be more invasive, but all arches will win.
>>
>>
>> Thank you very much for the examples, I think I understand what you are proposing.
>> I am not sure whether I'm able to map it to the mlx5 driver, but I could
>> try to come up with a RFC. It may take some time though.
>>
>> NVidia people, any suggesttions? Do you want to handle that yourselves?
>>
> Discussed with Saeed and he proposed another approach that is better for
> us: copy the whole skb payload inline into the WQE if it's size is below a
> threshold. This threshold can be configured through the
> tx-copybreak mechanism.
> 
> Thanks,
> Dragos


Thank you very much Dargos and Saeed.
I am not sure I understand the details of "inline into the WQE". 
The idea seems to be to use a premapped coherent array per WQ 
that is indexed by queue element index and can be used to copy headers and
maybe small messages into.
I think I see something similar to your proposal in mlx4 (?).
To me the general concept seems to be similar to what Eric is proposing.
Did I get it right?

I really like the idea to use tx-copybreak for threshold configuration.

As Eric mentioned that is not a very small patch and maybe not fit for backporting
to older distro versions.
What do you think of a two-step approach as described in the other sub-thread?
A simple patch for mitigation that can be backported, and then the improvement
as a replacement?

Thanks,
Alexandra