linux-kernel - Re: [PATCH net-next v4 0/4] TUN/TAP & vhost_net: netdev queue flow control to avoid ptr

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Message-ID: <CACGkMEviyLXU46YE=FmON-VomyWUtmjevE8FOFq=wwvjsmVoQQ@mail.gmail.com>
Date: Wed, 3 Sep 2025 12:00:40 +0800
From: Jason Wang <jasowang@...hat.com>
To: Simon Schippers <simon.schippers@...dortmund.de>
Cc: willemdebruijn.kernel@...il.com, mst@...hat.com, eperezma@...hat.com, 
	stephen@...workplumber.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, virtualization@...ts.linux.dev, 
	kvm@...r.kernel.org
Subject: Re: [PATCH net-next v4 0/4] TUN/TAP & vhost_net: netdev queue flow
 control to avoid ptr_ring tail drop

On Tue, Sep 2, 2025 at 4:10 PM Simon Schippers
<simon.schippers@...dortmund.de> wrote:
>
> This patch series deals with TUN/TAP and vhost_net which drop incoming
> SKBs whenever their internal ptr_ring buffer is full. Instead, with this
> patch series, the associated netdev queue is stopped before this happens.
> This allows the connected qdisc to function correctly as reported by [1]
> and improves application-layer performance, see benchmarks.
>
> This patch series includes TUN, TAP, and vhost_net because they share
> logic. Adjusting only one of them would break the others. Therefore, the
> patch series is structured as follows:
> 1. New ptr_ring_spare helper to check if the ptr_ring has spare capacity
> 2. Netdev queue flow control for TUN: Logic for stopping the queue upon
> full ptr_ring and waking the queue if ptr_ring has spare capacity
> 3. Additions for TAP: Similar logic for waking the queue
> 4. Additions for vhost_net: Calling TUN/TAP methods for waking the queue
>
> Benchmarks ([2] & [3]):
> - TUN: TCP throughput over real-world 120ms RTT OpenVPN connection
> improved by 36% (117Mbit/s vs 185 Mbit/s)
> - TAP: TCP throughput to local qemu VM stays the same (2.2Gbit/s), an
> improvement by factor 2 at emulated 120ms RTT (98Mbit/s vs 198Mbit/s)
> - TAP+vhost_net: TCP throughput to local qemu VM approx. the same
> (23.4Gbit/s vs 23.9Gbit/s), same performance at emulated 120ms RTT
> (200Mbit/s)
> - TUN/TAP/TAP+vhost_net: Reduction of ptr_ring size to ~10 packets
> possible without losing performance
>
> Possible future work:
> - Introduction of Byte Queue Limits as suggested by Stephen Hemminger
> - Adaption of the netdev queue flow control for ipvtap & macvtap

Could you please run pktgen on TUN as well to see the difference?

Thanks