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From: Eric Dumazet <eric.dumazet@gmail.com>
To: David Miller <davem@davemloft.net>
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Date: Fri, 29 Jun 2012 07:50:08 +0200
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Cc: nanditad@google.com, netdev@vger.kernel.org, codel@lists.bufferbloat.net,
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Subject: Re: [Codel] [PATCH net-next] fq_codel: report congestion
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On Thu, 2012-06-28 at 22:29 -0700, David Miller wrote:

> I am pretty sure the behavior in RED is intentional.
> 
> It's a soft push back on TCP.
> 

tcp_enter_cwr() is called the same for DROP and CN

> We're taking this path when we are unable to sucessfully ECN mark a
> packet.  But our intention was to do so.
> 

Hmm, problem is the sender thinks the packet was queued for
transmission.

        ret = macvlan_queue_xmit(skb, dev);
        if (likely(ret == NET_XMIT_SUCCESS || ret == NET_XMIT_CN)) {
                struct macvlan_pcpu_stats *pcpu_stats;

                pcpu_stats = this_cpu_ptr(vlan->pcpu_stats);
                u64_stats_update_begin(&pcpu_stats->syncp);
                pcpu_stats->tx_packets++;
                pcpu_stats->tx_bytes += len;
                u64_stats_update_end(&pcpu_stats->syncp);
        } else {
                this_cpu_inc(vlan->pcpu_stats->tx_dropped);
        }

NET_XMIT_CN has a lazy semantic it seems.

I will just dont rely on it.