netdev - Re: [RFC PATCH net-next 0/2] net: Use SMP threads for backlog NAPI.

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Date: Thu, 17 Aug 2023 08:30:25 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
 <pabeni@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner
 <tglx@...utronix.de>, Wander Lairson Costa <wander@...hat.com>, Yan Zhai
 <yan@...udflare.com>, Jesper Dangaard Brouer <hawk@...nel.org>, Daniel
 Borkmann <daniel@...earbox.net>, Martin KaFai Lau <martin.lau@...ux.dev>
Subject: Re: [RFC PATCH net-next 0/2] net: Use SMP threads for backlog NAPI.

On Thu, 17 Aug 2023 15:16:12 +0200 Sebastian Andrzej Siewior wrote:
> I've been looking at veth. In the xdp case it has its own NAPI instance.
> In the non-xdp it uses backlog. This should be called from
> ndo_start_xmit and user's write() so BH is off and interrupts are
> enabled at this point and it should be kind of rate-limited. Couldn't we
> bypass backlog in this case and deliver the packet directly to the
> stack?

The backlog in veth eats measurable percentage points of RPS of real
workloads, and I think number of people looked at getting rid of it.
So worthy goal for sure, but may not be a trivial fix.

To my knowledge the two main problems are:
 - we don't want to charge the sending application the processing for
   both "sides" of the connection and all the switching costs.
 - we may get an AA deadlock if the packet ends up looping in any way.

Or at least that's what I remember the problem being at 8am in the
morning :) Adding Daniel and Martin to CC, Paolo would also know this
better than me but I think he's AFK for the rest of the week.