netdev - Re: Hardcoded multicast queue length in macvlan.c driver causes poor multicast receive performance

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Date:   Thu, 26 Nov 2020 21:00:46 +0100
From:   Thomas Karlsson <thomas.karlsson@...eda.se>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Hardcoded multicast queue length in macvlan.c driver causes poor
 multicast receive performance

On 2020-11-26 00:01, Jakub Kicinski wrote:
> On Wed, 25 Nov 2020 23:15:39 +0100 Thomas Karlsson wrote:
>>>> This is my first time ever attemting a contribution to the kernel so
>>>> I'm quite happy to keep it simple like that too :)  
>>>
>>> Module params are highly inflexible, we have a general policy not 
>>> to accept them in the netdev world.
>>
>> I see, although the current define seems even less flexible :)
> 
> Just to be clear - the module parameter is a no-go. 
> No point discussing it.

Got it!

> 
>> Although, I might not have fully understood the .changelink you suggest.
>> Is it via the ip link set ... command? 
> 
> Yes.
> 
>> Or is there a way to set the parameters in a more "raw" form that
>> does not require a patch to iproute2 with parameter parsing, error
>> handing, man pages updates, etc. I feel that I'm getting in over my
>> head here.
> 
> We're here to assist! Netlink takes a little bit of effort 
> to comprehend but it's very simple once you get the mechanics!
> 

Thanks for the encouragement, I have been able to build iproute2 today and
I am successfully communicating with the driver now being able to set and retrieve my queue len!

As I'm working on this I do got a question. I placed the bc_queue_len into the struct macvlan_port *port
since that is where the bc_queue is located today. But when I change and retrieve the queue from userspace I realize
that all macvlan interfaces that share the same physical lowerdev uses the same port structure and thus
the same bc_queue_len.

It confused me at first and I'm not sure if that is how it should be. I expected the driver to have different
bc_queues for all macvlan interfaces no matter which lowerdev they were using but obviously that is not the case.

It may be a bit confusing to change bc_queue_len on one macvlan and see that the change was applied to more than one.

But I'm not sure if I should just move bc_queue_len to the struct macvlan_dev either. because then different macvlans will use different queue lengths while they still use the same queue. Which may also be considered a bit illogical

Let me know what you prefer here!

Thanks!