netdev - Re: [RFC net-next v4 0/9] Add support for per-NAPI config via netlink

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Message-ID: <Zv8o4eliTO60odQe@mini-arch>
Date: Thu, 3 Oct 2024 16:29:37 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Joe Damato <jdamato@...tly.com>
Cc: netdev@...r.kernel.org, mkarsten@...terloo.ca, skhawaja@...gle.com,
	sdf@...ichev.me, bjorn@...osinc.com, amritha.nambiar@...el.com,
	sridhar.samudrala@...el.com, willemdebruijn.kernel@...il.com,
	Alexander Lobakin <aleksander.lobakin@...el.com>,
	Breno Leitao <leitao@...ian.org>,
	Daniel Jurgens <danielj@...dia.com>,
	David Ahern <dsahern@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Donald Hunter <donald.hunter@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	"moderated list:INTEL ETHERNET DRIVERS" <intel-wired-lan@...ts.osuosl.org>,
	Jakub Kicinski <kuba@...nel.org>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	Jiri Pirko <jiri@...nulli.us>,
	Johannes Berg <johannes.berg@...el.com>,
	Jonathan Corbet <corbet@....net>,
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	Leon Romanovsky <leon@...nel.org>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:MELLANOX MLX4 core VPI driver" <linux-rdma@...r.kernel.org>,
	Lorenzo Bianconi <lorenzo@...nel.org>,
	Michael Chan <michael.chan@...adcom.com>,
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	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
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	Tariq Toukan <tariqt@...dia.com>,
	Tony Nguyen <anthony.l.nguyen@...el.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [RFC net-next v4 0/9] Add support for per-NAPI config via netlink

On 10/01, Joe Damato wrote:
> Greetings:
> 
> Welcome to RFC v4.
> 
> Very important and significant changes have been made since RFC v3 [1],
> please see the changelog below for details.
> 
> A couple important call outs for this revision for reviewers:
> 
>   1. idpf embeds a napi_struct in an internal data structure and
>      includes an assertion on the size of napi_struct. The maintainers
>      have stated that they think anyone touching napi_struct should update
>      the assertion [2], so I've done this in patch 3. 
> 
>      Even though the assertion has been updated, I've given the
>      cacheline placement of napi_struct within idpf's internals no
>      thought or consideration.
> 
>      Would appreciate other opinions on this; I think idpf should be
>      fixed. It seems unreasonable to me that anyone changing the size of
>      a struct in the core should need to think about cachelines in idpf.

[..]

>   2. This revision seems to work (see below for a full walk through). Is
>      this the behavior we want? Am I missing some use case or some
>      behavioral thing other folks need?

The walk through looks good!


>   3. Re a previous point made by Stanislav regarding "taking over a NAPI
>      ID" when the channel count changes: mlx5 seems to call napi_disable
>      followed by netif_napi_del for the old queues and then calls
>      napi_enable for the new ones. In this RFC, the NAPI ID generation
>      is deferred to napi_enable. This means we won't end up with two of
>      the same NAPI IDs added to the hash at the same time (I am pretty
>      sure).


[..]

>      Can we assume all drivers will napi_disable the old queues before
>      napi_enable the new ones? If yes, we might not need to worry about
>      a NAPI ID takeover function.

With the explicit driver opt-in via netif_napi_add_config, this
shouldn't matter? When somebody gets to converting the drivers that
don't follow this common pattern they'll have to solve the takeover
part :-)