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

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Message-ID: <20201125100710.7e766d7e@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 25 Nov 2020 10:07:10 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Thomas Karlsson <thomas.karlsson@...eda.se>
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 Wed, 25 Nov 2020 18:12:34 +0100 Thomas Karlsson wrote:
> >> For this reason I would like to know if you would consider
> >> merging a patch using the module_param(...) variant instead?
> >>
> >> I would argue that this still makes the situation better
> >> and resolves the packet-loss issue, although not necessarily
> >> in an optimal way. However, The upside of being able to specify the
> >> parameter on a per macvlan interface level instead of globally is not
> >> that big in this situation. Normally you don't use that much
> >> multicast anyway so it's a parameter that only will be touched by
> >> a very small user base that can understand and handle the implications
> >> of such a global setting.  
> > 
> > How about implementing .changelink in macvlan? That way you could
> > modify the macvlan device independent of Docker? 
> > 
> > Make sure you only accept changes to the bc queue len if that's the
> > only one you act on.
> >   
> 
> Hmm, I see. You mean that docker can create the interface and then I can
> modify it afterwards? That might be a workaround but I just submitted
> a patch (like seconds before your message) with the module_param() option
> and this was very clean I think. both in how little code that needed to be
> changed and in how simple it is to set the option in the target environment.
> 
> 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. There should even be a check 
in our patchwork which should fail here, but it appears that the patch 
did not apply in the first place:

https://patchwork.kernel.org/project/netdevbpf/patch/385b9b4c-25f5-b507-4e69-419883fa8043@paneda.se/

Make sure you're developing on top of this tree:

https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/