netdev - Re: [Question] About bonding offload

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Date:   Mon, 10 Apr 2023 21:58:20 -0700
From:   Jay Vosburgh <jay.vosburgh@...onical.com>
To:     Liang Li <liali@...hat.com>
cc:     vfalico@...il.com, andy@...yhouse.net, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org,
        Paolo Abeni <pabeni@...hat.com>, ast@...nel.org,
        daniel@...earbox.net, hawk@...nel.org, john.fastabend@...il.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Hangbin Liu <haliu@...hat.com>,
        "Toppins, Jonathan" <jtoppins@...hat.com>
Subject: Re: [Question] About bonding offload

Liang Li <liali@...hat.com> wrote:

>Hi Everyone,
>
>I'm a redhat network-qe and am testing bonding offload. e.g. gso,tso,gro,lro.
>I got two questions during my testing.
>
>1. The tcp performance has no difference when bonding GRO is on versus off.
>When testing with bonding, I always get ~890 Mbits/sec bandwidth no
>matter whether GRO is on.
>When testing with a physical NIC instead of bonding on the same
>machine, with GRO off, I get 464 Mbits/sec bandwidth, with GRO on, I
>get  897 Mbits/sec bandwidth.
>So looks like the GRO can't be turned off on bonding?

	Well, it's probably more correct to say that GRO is
unimplemented for "stacked on top" interfaces like bonding (or bridge,
vlan, team, etc).  GRO operates early in the receive processing, when
the device driver is receiving packets, typically by calling
napi_gro_receive() from its NAPI poll function.  This is well before
bonding, bridge, et al, are involved, as these drivers don't do NAPI at
all.

>I used iperf3 to test performance.
>And I limited iperf3 process cpu usage during my testing to simulate a
>cpu bottleneck.
>Otherwise it's difficult to see bandwidth differences when offload is
>on versus off.
>
>I reported a bz for this: https://bugzilla.redhat.com/show_bug.cgi?id=2183434
>
>2.  Should bonding propagate offload configuration to slaves?
>For now, only "ethtool -K bond0 lro off" can be propagated to slaves,
>others can't be propagated to slaves, e.g.
>  ethtool -K bond0 tso on/off
>  ethtool -K bond0 gso on/off
>  ethtool -K bond0 gro on/off
>  ethtool -K bond0 lro on
>All above configurations can't be propagated to bonding slaves.

	The LRO case is because it's set in NETIF_F_UPPER_DISABLES, as
checked in netdev_sync_upper_features() and netdev_sync_lower_features().

	A subset of features is handled in bond_compute_features().
Some feature changes, e.g., scatter-gather, do propagate upwards (but
not downwards), as bonding handles NETDEV_FEAT_CHANGE events for its
members (but not vice versa).

	TSO, GSO, and GRO aren't handled in either of these situations,
and so changes don't propagate at all.  Whether they should or not is a
separate, complicated, question.  E.g., should features propagate
upwards, or downwards?  How many levels of nesting?

	-J

>I reports a bz for this: https://bugzilla.redhat.com/show_bug.cgi?id=2183777
>
>I am using the RHEL with kernel 4.18.0-481.el8.x86_64.
>
>BR,
>Liang Li
>

---
	-Jay Vosburgh, jay.vosburgh@...onical.com