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

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Message-ID: <ZwAWQCpSbb4sn0LX@mini-arch>
Date: Fri, 4 Oct 2024 09:22:24 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Joe Damato <jdamato@...tly.com>, 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>,
	Kory Maincent <kory.maincent@...tlin.com>,
	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>,
	Mina Almasry <almasrymina@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Saeed Mahameed <saeedm@...dia.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	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 10/03, Joe Damato wrote:
> On Thu, Oct 03, 2024 at 04:53:13PM -0700, Joe Damato wrote:
> > On Thu, Oct 03, 2024 at 04:29:37PM -0700, Stanislav Fomichev wrote:
> > > On 10/01, Joe Damato wrote:
> > 
> > [...]
> >  
> > > >   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!
> > 
> > Thanks for taking a look.
> > 
> > > >   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 :-)
> > 
> > That is true; that's a good point.
> 
> Actually, sorry, that isn't strictly true. NAPI ID generation is
> moved for everything to napi_enable; they just are (or are not)
> persisted depending on whether the driver opted in to add_config or
> not.
> 
> So, the change does affect all drivers. NAPI IDs won't be generated
> and added to the hash until napi_enable and they will be removed
> from the hash in napi_disable... even if you didn't opt-in to having
> storage.
> 
> Opt-ing in to storage via netif_napi_add_config just means that your
> NAPI IDs (and other settings) will be persistent.
> 
> Sorry about my confusion when replying earlier.

AFAIA, all control operations (ethtool or similar ones via netlink),
should grab rtnl lock. So as long as both enable/disable happen
under rtnl (and in my mind they should), I don't think there is gonna
be any user-visible side-effects of your change. But I might be wrong,
let's see if others can come up with some corner cases..