netdev - Re: [PATCH bpf-next 3/6] xsk: introduce xsk_do_redirect_rx

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Message-ID: <dfa75afc-ceb7-76ce-6ba3-3b89c53f92f3@intel.com>
Date:   Fri, 4 Sep 2020 17:39:17 +0200
From:   Björn Töpel <bjorn.topel@...el.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Björn Töpel <bjorn.topel@...il.com>
Cc:     ast@...nel.org, daniel@...earbox.net, netdev@...r.kernel.org,
        bpf@...r.kernel.org, magnus.karlsson@...el.com,
        davem@...emloft.net, kuba@...nel.org, hawk@...nel.org,
        john.fastabend@...il.com, intel-wired-lan@...ts.osuosl.org
Subject: Re: [PATCH bpf-next 3/6] xsk: introduce xsk_do_redirect_rx_full()
 helper

On 2020-09-04 17:11, Jesper Dangaard Brouer wrote:
> On Fri,  4 Sep 2020 15:53:28 +0200 Björn Töpel
> <bjorn.topel@...il.com> wrote:
> 
>> From: Björn Töpel <bjorn.topel@...el.com>
>> 
>> 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?

Björn