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Message-ID: <6677dc5cb5cca_33522729474@willemb.c.googlers.com.notmuch>
Date: Sun, 23 Jun 2024 04:27:08 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Yan Zhai <yan@...udflare.com>,
Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Jesper Dangaard Brouer <hawk@...nel.org>,
John Fastabend <john.fastabend@...il.com>,
Willem de Bruijn <willemb@...gle.com>,
Simon Horman <horms@...nel.org>,
Florian Westphal <fw@...len.de>,
Mina Almasry <almasrymina@...gle.com>,
Abhishek Chauhan <quic_abchauha@...cinc.com>,
David Howells <dhowells@...hat.com>,
Alexander Lobakin <aleksander.lobakin@...el.com>,
David Ahern <dsahern@...nel.org>,
Richard Gobert <richardbgobert@...il.com>,
Antoine Tenart <atenart@...nel.org>,
Felix Fietkau <nbd@....name>,
Soheil Hassas Yeganeh <soheil@...gle.com>,
Pavel Begunkov <asml.silence@...il.com>,
Lorenzo Bianconi <lorenzo@...nel.org>,
Thomas Weißschuh <linux@...ssschuh.net>,
linux-kernel@...r.kernel.org,
bpf@...r.kernel.org
Subject: Re: [RFC net-next 1/9] skb: introduce gro_disabled bit
Yan Zhai wrote:
> > > -static inline bool netif_elide_gro(const struct net_device *dev)
> > > +static inline bool netif_elide_gro(const struct sk_buff *skb)
> > > {
> > > - if (!(dev->features & NETIF_F_GRO) || dev->xdp_prog)
> > > + if (!(skb->dev->features & NETIF_F_GRO) || skb->dev->xdp_prog)
> > > return true;
> > > +
> > > +#ifdef CONFIG_SKB_GRO_CONTROL
> > > + return skb->gro_disabled;
> > > +#else
> > > return false;
> > > +#endif
> >
> > Yet more branches in the hot path.
> >
> > Compile time configurability does not help, as that will be
> > enabled by distros.
> >
> > For a fairly niche use case. Where functionality of GRO already
> > works. So just a performance for a very rare case at the cost of a
> > regression in the common case. A small regression perhaps, but death
> > by a thousand cuts.
> >
>
> I share your concern on operating on this hotpath. Will a
> static_branch + sysctl make it less aggressive?
That is always a possibility. But we have to use it judiciously,
cannot add a sysctl for every branch.
I'm still of the opinion that Paolo shared that this seems a lot of
complexity for a fairly minor performance optimization for a rare
case.
> Speaking of
> performance, I'd hope this can give us more control so we can achieve
> the best of two worlds: for TCP and some UDP traffic, we can enable
> GRO, while for some other classes that we know GRO does no good or
> even harm, let's disable GRO to save more cycles. The key observation
> is that developers may already know which traffic is blessed by GRO,
> but lack a way to realize it.
Following up also on Daniel's point on using BPF as GRO engine. Even
earlier I tried to add an option to selectively enable GRO protocols
without BPF. Definitely worthwhile to be able to disable GRO handlers
to reduce attack surface to bad input.
>
> best
> Yan
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