netdev - Re: [PATCH net-next v2 2/4] sch_htb: Hierarchical QoS hardware offload

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Date:   Thu, 17 Dec 2020 10:09:17 -0500
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Maxim Mikityanskiy <maximmi@...dia.com>,
        Cong Wang <xiyou.wangcong@...il.com>
Cc:     Maxim Mikityanskiy <maximmi@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jiri Pirko <jiri@...nulli.us>,
        Saeed Mahameed <saeedm@...dia.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Yossi Kuperman <yossiku@...dia.com>
Subject: Re: [PATCH net-next v2 2/4] sch_htb: Hierarchical QoS hardware
 offload

On 2020-12-16 6:47 a.m., Maxim Mikityanskiy wrote:
> On 2020-12-15 18:37, Jamal Hadi Salim wrote:

[..]

>>
>> Same question above:
>> Is there a limit to the number of classes that can be created?
> 
> Yes, the commit message of the mlx5 patch lists the limitations of our 
> NICs. Basically, it's 256 leaf classes and 3 levels of hierarchy.
>

Ok, thats what i was looking for.


>> IOW, if someone just created an arbitrary number of queues do they
>> get errored-out if it doesnt make sense for the hardware?
> 
> The current implementation starts failing gracefully if the limits are 
> exceeded. The tc command won't succeed, and everything will roll back to 
> the stable state, which was just before the tc command.
>

Does the user gets notified somehow or it fails silently?
An extack message would help.


>> If such limits exist, it may make sense to provide a knob to query
>> (maybe ethtool)
> 
> Sounds legit, but I'm not sure what would be the best interface for 
> that. Ethtool is not involved at all in this implementation, and AFAIK 
> it doesn't contain any existing command for similar stuff. We could hook 
> into set-channels and add new type of channels for HTB, but the 
> semantics isn't very clear, because HTB queues != HTB leaf classes, and 
> I don't know if it's allowed to extend this interface (if so, I have 
> more thoughts of extending it for other purposes).
> 

More looking to make sure no suprise to the user. Either the user can
discover what the constraints are or when they provision they get a
a message like "cannot offload more than 3 hierarchies" or "use devlink
if you want to use more than 256 classes", etc.

cheers,
jamal