netdev - Re: Throughput regression with `tcp: refine TSO autosizing`

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Date:	Thu, 05 Feb 2015 05:19:02 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michal Kazior <michal.kazior@...to.com>
Cc:	Neal Cardwell <ncardwell@...gle.com>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	eyalpe@....mellanox.co.il
Subject: Re: Throughput regression with `tcp: refine TSO autosizing`

On Thu, 2015-02-05 at 04:57 -0800, Eric Dumazet wrote:

> The intention is to control the queues to the following :
> 
> 1 ms of buffering, but limited to a configurable value.
> 
> On a 40Gbps flow, 1ms represents 5 MB, which is insane.
> 
> We do not want to queue 5 MB of traffic, this would destroy latencies
> for all concurrent flows. (Or would require having fq_codel or fq as
> packet schedulers, instead of default pfifo_fast)
> 
> This is why having 1.5 ms delay between the transmit and TX completion
> is a problem in your case.

Note that TCP stack could detect when this happens, *if* ACK where
delivered before the TX completions, or when TX completion happens,
we could detect that the clone of the freed packet was freed.

In my test, when I did "ethtool -C eth0 tx-usecs 1024 tx-frames 64", and
disabling GSO, TCP stack sends a bunch of packets (a bit less than 64),
blocks on tcp_limit_output_bytes.

Then we receive 2 stretch ACKS after ~50 usec.

TCP stack tries to push again some packets but blocks on
tcp_limit_output_bytes again.

1ms later, TX completion happens, tcp_wfree() is called, and TCP stack
push following ~60 packets.

TCP could  eventually dynamically adjust the tcp_limit_output_bytes,
using a per flow dynamic value, but I would rather not add a kludge in
TCP stack only to deal with a possible bug in ath10k driver.

niu has a similar issue and simply had to call skb_orphan() :

drivers/net/ethernet/sun/niu.c:6669:            skb_orphan(skb);

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