netdev - mlx4: Bug in XDP_TX + 16 rx-queues

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Date:   Sat, 17 Dec 2016 02:18:03 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Saeed Mahameed <saeedm@...lanox.com>,
        Tariq Toukan <tariqt@...lanox.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>
Subject: mlx4: Bug in XDP_TX + 16 rx-queues

Hi All,

I have been debugging with XDP_TX and 16 rx-queues.

1) When 16 rx-queues is used and an XDP prog is doing XDP_TX,
it seems that the packet cannot be XDP_TX out if the pkt
is received from some particular CPUs (/rx-queues).

2) If 8 rx-queues is used, it does not have problem.

3) The 16 rx-queues problem also went away after reverting these
two patches:
15fca2c8eb41 net/mlx4_en: Add ethtool statistics for XDP cases
67f8b1dcb9ee net/mlx4_en: Refactor the XDP forwarding rings scheme

4) I can reproduce the problem by running samples/bof/xdp_ip_tunnel at
the receiver side.  The sender side sends out TCP packets with
source port ranging from 1 to 1024.  At the sender side also, do
a tcpdump to capture the ip-tunnel packet reflected by xdp_ip_tunnel.
With 8 rx-queues,  I can get all 1024 packets back.  With 16 rx-queues,
I can only get 512 packets back.  It is a 40 CPUs machine.
I also checked the rx*_xdp_tx counters (from ethtool -S eth0) to ensure
the xdp prog has XDP_TX-ed it out.

Not saying that 67f8b1dcb9ee is 100% the cause because there are other
changes since then.  It is merely a brain dump on what I have already
tried.

Tariq/Saeed, any thoughts?  I can easily test some patches in
my setup.

Thanks,
--Martin