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Date:   Fri, 4 Sep 2020 17:39:17 +0200
From:   Björn Töpel <>
To:     Jesper Dangaard Brouer <>,
        Björn Töpel <>
Subject: Re: [PATCH bpf-next 3/6] xsk: introduce xsk_do_redirect_rx_full()

On 2020-09-04 17:11, Jesper Dangaard Brouer wrote:
> On Fri,  4 Sep 2020 15:53:28 +0200 Björn Töpel
> <> wrote:
>> From: Björn Töpel <>
>> The xsk_do_redirect_rx_full() helper can be used to check if a
>> failure of xdp_do_redirect() was due to the AF_XDP socket had a
>> full Rx ring.
> This is very AF_XDP specific.  I think that the cpumap could likely 
> benefit from similar approach? e.g. if the cpumap kthread is
> scheduled on the same CPU.

At least I thought this was *very* AF_XDP specific, since the kernel is
dependent of that userland runs. Allocation (source) and Rx ring (sink).
Maybe I was wrong! :-)

The thing with AF_XDP zero-copy, is that we sort of assume that if a
user enabled that most packets will have XDP_REDIRECT to an AF_XDP socket.

> But for cpumap we only want this behavior if sched on the same CPU
> as RX-NAPI.  This could be "seen" by the cpumap code itself in the
> case bq_flush_to_queue() drops packets, check if rcpu->cpu equal 
> smp_processor_id().  Maybe I'm taking this too far?

Interesting. So, if you're running on the same core, and redirect fail
for CPUMAP, you'd like to yield the NAPI loop? Is that really OK from a
fairness perspective? I mean, with AF_XDP zero-copy we pretty much know
that all actions will be redirect to socket. For CPUMAP type of
applications, can that assumption be made?


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