netdev - Re: [PATCH net-next v4 0/3] add tx packets aggregation to ethtool and rmnet

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Date:   Thu, 12 Jan 2023 14:59:57 -0800
From:   Alexander H Duyck <alexander.duyck@...il.com>
To:     Daniele Palmas <dnlplm@...il.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Subash Abhinov Kasiviswanathan <quic_subashab@...cinc.com>,
        Sean Tranchetti <quic_stranche@...cinc.com>,
        Jonathan Corbet <corbet@....net>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Gal Pressman <gal@...dia.com>, Dave Taht <dave.taht@...il.com>
Cc:     Bjørn Mork <bjorn@...k.no>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        netdev@...r.kernel.org
Subject: Re: [PATCH net-next v4 0/3] add tx packets aggregation to ethtool
 and rmnet

On Wed, 2023-01-11 at 14:05 +0100, Daniele Palmas wrote:
> Hello maintainers and all,
> 
> this patchset implements tx qmap packets aggregation in rmnet and generic
> ethtool support for that.
> 
> Some low-cat Thread-x based modems are not capable of properly reaching the maximum
> allowed throughput both in tx and rx during a bidirectional test if tx packets
> aggregation is not enabled.

One question I would have about this is if you are making use of Byte
Queue Limits and netdev_xmit_more at all? I know for high speed devices
most of us added support for xmit_more because PCIe bandwidth was
limiting when we were writing the Tx ring indexes/doorbells with every
packet. To overcome that we added netdev_xmit_more which told us when
the Qdisc had more packets to give us. This allowed us to better
utilize the PCIe bus by bursting packets through without adding
additional latency.

> I verified this problem with rmnet + qmi_wwan by using a MDM9207 Cat. 4 based modem
> (50Mbps/150Mbps max throughput). What is actually happening is pictured at
> https://drive.google.com/file/d/1gSbozrtd9h0X63i6vdkNpN68d-9sg8f9/view
> 
> Testing with iperf TCP, when rx and tx flows are tested singularly there's no issue
> in tx and minor issues in rx (not able to reach max throughput). When there are concurrent
> tx and rx flows, tx throughput has an huge drop. rx a minor one, but still present.
> 
> The same scenario with tx aggregation enabled is pictured at
> https://drive.google.com/file/d/1jcVIKNZD7K3lHtwKE5W02mpaloudYYih/view
> showing a regular graph.
> 
> This issue does not happen with high-cat modems (e.g. SDX20), or at least it
> does not happen at the throughputs I'm able to test currently: maybe the same
> could happen when moving close to the maximum rates supported by those modems.
> Anyway, having the tx aggregation enabled should not hurt.
> 
> The first attempt to solve this issue was in qmi_wwan qmap implementation,
> see the discussion at https://lore.kernel.org/netdev/20221019132503.6783-1-dnlplm@gmail.com/
> 
> However, it turned out that rmnet was a better candidate for the implementation.
> 
> Moreover, Greg and Jakub suggested also to use ethtool for the configuration:
> not sure if I got their advice right, but this patchset add also generic ethtool
> support for tx aggregation.

I have concerns about this essentially moving queueing disciplines down
into the device. The idea of doing Tx aggregation seems like something
that should be done with the qdisc rather than the device driver.
Otherwise we are looking at having multiple implementations of this
aggregation floating around. It seems like it would make more sense to
have this batching happen at the qdisc layer, and then the qdisc layer
would pass down a batch of frames w/ xmit_more set to indicate it is
flushing the batch.