netdev - Re: [PATCH RFC] net/mlx5_en: switch to Toeplitz RSS hash by default

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Date:   Thu, 6 Sep 2018 13:04:45 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Saeed Mahameed <saeedm@....mellanox.co.il>
Cc:     Tariq Toukan <tariqt@...lanox.com>,
        Linux Netdev List <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Gal Pressman <galp@...lanox.com>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH RFC] net/mlx5_en: switch to Toeplitz RSS hash by default



On 06.09.2018 08:24, Saeed Mahameed wrote:
> On Sun, Sep 2, 2018 at 2:55 AM, Konstantin Khlebnikov
> <khlebnikov@...dex-team.ru> wrote:
>> On 02.09.2018 12:29, Tariq Toukan wrote:
>>>
>>>
>>>
>>> On 31/08/2018 2:29 PM, Konstantin Khlebnikov wrote:
>>>>
>>>> XOR (MLX5_RX_HASH_FN_INVERTED_XOR8) gives only 8 bits.
>>>> It seems not enough for RFS. All other drivers use toeplitz.
>>>>
>>>> Driver mlx4_en uses Toeplitz by default and warns if hash XOR is used
>>>> together with NETIF_F_RXHASH (enabled by default too): "Enabling both
>>>> XOR Hash function and RX Hashing can limit RPS functionality".
>>>>
>>>> XOR is default in mlx5_en since commit 2be6967cdbc9
>>>> ("net/mlx5e: Support ETH_RSS_HASH_XOR").
>>>>
>>>> Hash function could be set via ethtool. But it would be nice to have
>>>> single standard for drivers or proper description why this one is
>>>> special.
>>>>
>>>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
>>>> ---
>>>
>>>
>>> Hi Konstantin,
>>>
>>> Thanks for the patch.
>>>
>>> I understand the motivation.
>>>
>>> This change affects the default out-of-the-box behavior and requires a
>>> full performance cycle. We'll run performance regression tomorrow, results
>>> should be ready by EOW.
>>>   > I'll update.
>>
>>
>> Ok, thank you.
>>
>> The only mention I've found in your documentation
>> http://www.mellanox.com/related-docs/prod_software/Mellanox_EN_for_Linux_User_Manual_v4_4.pdf
>>
>> is
>> ---
>> 1.1.10 RSS Support
>> 1.1.10.1 RSS Hash Function
>> The device has the ability to use XOR as the RSS distribution function,
>> instead of the default
>> Toplitz function.
>> The XOR function can be better distributed among driver's receive queues in
>> small number of
>> streams, where it distributes each TCP/UDP stream to a different queue.
>> ---
>>
>> So Toeplitz is supposed to be default hash function for all versions of
>> drivers and hardware.
>>
>> Also XOR8 seems vulnerable for ddos - hash is predictable, no random\secret
>> vector, only 8 bits.
>> So, it's easy to route all flows into one point. As we got it by accident.
>>
>> Moreover, in kernel 4.4.y hash switch via ethtool is broken and does not
>> work =)
>>
> 
> is it broken in mlx5 only or for the whole kernel ?

In mlx5 only - interface works but makes no effect.

For now we have disabled RXHASH as workaround.

> 
> If it is mlx5 then this might be the reason:
> commit 2d75b2bc8a8c0ce5567a6ecef52e194d117efe3f
> net/mlx5e: Add ethtool RSS configuration options
> 
> was submitted to kernel 4.3
> 
> and an important fix for hash function change was submitted to 4.5:
> 
> commit bdfc028de1b3cd59490d5413a5c87b0fa50040c2
> Author: Tariq Toukan <tariqt@...lanox.com>
> Date:   Mon Feb 29 21:17:12 2016 +0200
> 
>      net/mlx5e: Fix ethtool RX hash func configuration change
> 
>      We should modify TIRs explicitly to apply the new RSS configuration.
>      The light ndo close/open calls do not "refresh" them.
> 
>      Fixes: 2d75b2bc8a8c ('net/mlx5e: Add ethtool RSS configuration options')
> 

I think so, but this patch also seems flawed and fixed in

commit a100ff3eef193d2d79daf98dcd97a54776ffeb78
Author: Gal Pressman <galp@...lanox.com>
Date:   Thu Jan 12 16:25:46 2017 +0200

     net/mlx5e: Fix update of hash function/key via ethtool

     Modifying TIR hash should change selected fields bitmask in addition to
     the function and key.

And maybe more, there a lot of progress since v4.4

> 
>>>
>>> Regards,
>>> Tariq