netdev - Re: [PATCH bpf-next 03/12] net: Add IFLA_XDP_EGRESS for XDP programs in the egress path

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Date:   Thu, 30 Jan 2020 06:45:17 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc:     David Ahern <dsahern@...il.com>,
        Toke Høiland-Jørgensen 
        <toke@...hat.com>, David Ahern <dsahern@...nel.org>,
        netdev@...r.kernel.org, prashantbhole.linux@...il.com,
        jasowang@...hat.com, davem@...emloft.net, mst@...hat.com,
        toshiaki.makita1@...il.com, daniel@...earbox.net,
        john.fastabend@...il.com, ast@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, andriin@...com,
        David Ahern <dahern@...italocean.com>
Subject: Re: [PATCH bpf-next 03/12] net: Add IFLA_XDP_EGRESS for XDP
 programs in the egress path

On Tue, 28 Jan 2020 15:13:43 +0100, Jesper Dangaard Brouer wrote:
> On Sun, 26 Jan 2020 14:17:01 -0800
> Jakub Kicinski <kuba@...nel.org> wrote:
> 
> > On Sun, 26 Jan 2020 13:49:33 +0100, Jesper Dangaard Brouer wrote:  
> > > Yes, please. I want this NIC TX hook to see both SKBs and xdp_frames.    
> > 
> > Any pointers on what for? Unless we see actual use cases there's
> > a justifiable concern of the entire thing just being an application of
> > "We can solve any problem by introducing an extra level of indirection."  
> 
> I have two use-cases:
> 
> (1) For XDP easier handling of interface specific setting on egress,
> e.g. pushing a VLAN-id, instead of having to figure this out in RX hook.
> (I think this is also David Ahern's use-case)

Is it really useful to have a hook before multi-buffer frames are
possible and perhaps TSO? The local TCP performance is going to tank
with XDP enabled otherwise.

> (2) I want this egress XDP hook to have the ability to signal
> backpressure. Today we have BQL in most drivers (which is essential to
> avoid bufferbloat). For XDP_REDIRECT we don't, which we must solve.
> 
> For use-case(2), we likely need a BPF-helper calling netif_tx_stop_queue(),
> or a return code that can stop the queue towards the higher layers.

Agreed, although for that use case, I'm not sure if non-XDP frames 
have to pass trough the hook. Hard to tell as the current patches 
don't touch on this use case.