netdev - Re: [PATCH net-next v2] ethtool: add netlink based get rxfh support

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Date:   Sun, 13 Nov 2022 22:23:25 -0600
From:   "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     "Mogilappagari, Sudheer" <sudheer.mogilappagari@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "mkubecek@...e.cz" <mkubecek@...e.cz>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "corbet@....net" <corbet@....net>,
        "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>
Subject: Re: [PATCH net-next v2] ethtool: add netlink based get rxfh support

On 11/10/2022 6:12 PM, Jakub Kicinski wrote:
> On Thu, 10 Nov 2022 17:24:15 -0600 Samudrala, Sridhar wrote:
>>> The RSS context thing is a pretty shallow abstraction, I don't think we
>>> should be extending it into "queue groups" or whatnot. We'll probably
>>> need some devlink objects at some point (rate configuration?) and
>>> locking order is devlink > rtnl, so spawning things from within ethtool
>>> will be a pain :S
>> We are going this path of extending ethtool rss context interface to support
>> per queue-group parameters based on this feedback.
>>     https://lore.kernel.org/netdev/20220314131114.635d5acb@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com/
>>
>> Per queue-group rate can already be configured when creating queue groups via
>> tc-mpqrio interface using bw_rlimit parameters.
> Right, but that's still just flow-director-y/hash-y thing?

Yes. The initial per queue-group parameter we want to add is to make incoming
TCP connections to be distributed in a round-robin manner by over-riding RSS
hash based queue when a SYN is received and let a flow director rule added
when a SYN-ACK is sent out.

> Does the name RSS imply purely hash based distribution?

The name Receive Side Scaling doesn't explicitly say that the distribution
is hash based, but I think it is a general assumption that it is based on hash.
But if you are OK to use RSS prefix for flow-director based distributions,
we will use ethtool rss-context interface for this parameter.

>
> My worry is that if we go with a more broad name like
> "queue group" someone may be mislead to adding controls
> unrelated to flow <> queue assignment here.

Later we would like to add a per queue-group parameter that would allow
reducing/changing the number of napi pollers for a queue group from the default
value equal to the number of queues in the queue group. Are you suggesting
creating a queue-group object and use devlink API to configure such parameters
for a queue-group?