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Date: Thu, 17 Aug 2023 08:30:25 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, "David S. Miller"
 <davem@davemloft.net>, Eric Dumazet <edumazet@google.com>, Paolo Abeni
 <pabeni@redhat.com>, Peter Zijlstra <peterz@infradead.org>, Thomas Gleixner
 <tglx@linutronix.de>, Wander Lairson Costa <wander@redhat.com>, Yan Zhai
 <yan@cloudflare.com>, Jesper Dangaard Brouer <hawk@kernel.org>, Daniel
 Borkmann <daniel@iogearbox.net>, Martin KaFai Lau <martin.lau@linux.dev>
Subject: Re: [RFC PATCH net-next 0/2] net: Use SMP threads for backlog NAPI.
Message-ID: <20230817083025.2e8047fa@kernel.org>
In-Reply-To: <20230817131612.M_wwTr7m@linutronix.de>
References: <20230814093528.117342-1-bigeasy@linutronix.de>
	<20230814112421.5a2fa4f6@kernel.org>
	<20230817131612.M_wwTr7m@linutronix.de>
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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.