netdev - Re: [PATCH v3 bpf-next] cpumap: bulk skb using netif_receive_skb

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Date:   Thu, 22 Apr 2021 14:30:57 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>,
        BPF-dev-list <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Song Liu <song@...nel.org>,
        Toke Hoiland Jorgensen <toke@...hat.com>, brouer@...hat.com
Subject: Re: [PATCH v3 bpf-next] cpumap: bulk skb using
 netif_receive_skb_list

On Thu, 22 Apr 2021 12:59:31 +0200
Lorenzo Bianconi <lorenzo.bianconi@...hat.com> wrote:

> >
> > [...]  
> > > > +   TP_ARGS(map_id, processed, sched, xdp_stats),
> > > >
> > > >     TP_STRUCT__entry(
> > > >             __field(int, map_id)
> > > >             __field(u32, act)
> > > >             __field(int, cpu)
> > > > -           __field(unsigned int, drops)
> > > >             __field(unsigned int, processed)  
> > >
> > > So, struct member @processed will takeover the room for @drops.
> > >
> > > Can you please test how an old xdp_monitor program will react to this?
> > > Will it fail, or extract and show wrong values?  
> >
> > Ack, right. I think we should keep the struct layout in order to maintain
> > back-compatibility. I will fix it in v4.
> >  
> > >
> > > The xdp_mointor tool is in several external git repos:
> > >
> > >  https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/samples/bpf/xdp_monitor_kern.c
> > >  https://github.com/xdp-project/xdp-tutorial/tree/master/tracing02-xdp-monitor  
> 
> Running an old version of xdp_monitor with a patched kernel, I
> verified the xdp sample does not crash but it reports wrong values
> (e.g. pps are reported as drops for tracepoint disagliment).
> I think we have two possibilities here:
> - assuming tracepoints are not a stable ABI, we can just fix xdp
> samples available in the kernel tree and provide a patch for
> xdp-project
>
> - keep current tracepoint layout and just rename current drop variable
> in bpf/cpumap.c in something like skb_alloc_drop.

I like this one the most, rename to skb_alloc_drop, but keep layout.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer