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Message-ID: <CACGkMEtnLw2b8iDysQzRbXxTj2xbgzKqEZUbhmZz9tXPLTE6Sw@mail.gmail.com>
Date: Fri, 30 Jan 2026 09:51:32 +0800
From: Jason Wang <jasowang@...hat.com>
To: Simon Schippers <simon.schippers@...dortmund.de>
Cc: willemdebruijn.kernel@...il.com, andrew+netdev@...n.ch,
davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
mst@...hat.com, eperezma@...hat.com, leiyang@...hat.com,
stephen@...workplumber.org, jon@...anix.com, tim.gebauer@...dortmund.de,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
virtualization@...ts.linux.dev
Subject: Re: [PATCH net-next v7 3/9] tun/tap: add ptr_ring consume helper with
netdev queue wakeup
On Thu, Jan 29, 2026 at 5:25 PM Simon Schippers
<simon.schippers@...dortmund.de> wrote:
>
> On 1/29/26 02:14, Jason Wang wrote:
> > On Wed, Jan 28, 2026 at 3:54 PM Simon Schippers
> > <simon.schippers@...dortmund.de> wrote:
> >>
> >> On 1/28/26 08:03, Jason Wang wrote:
> >>> On Wed, Jan 28, 2026 at 12:48 AM Simon Schippers
> >>> <simon.schippers@...dortmund.de> wrote:
> >>>>
> >>>> On 1/23/26 10:54, Simon Schippers wrote:
> >>>>> On 1/23/26 04:05, Jason Wang wrote:
> >>>>>> On Thu, Jan 22, 2026 at 1:35 PM Jason Wang <jasowang@...hat.com> wrote:
> >>>>>>>
> >>>>>>> On Wed, Jan 21, 2026 at 5:33 PM Simon Schippers
> >>>>>>> <simon.schippers@...dortmund.de> wrote:
> >>>>>>>>
> >>>>>>>> On 1/9/26 07:02, Jason Wang wrote:
> >>>>>>>>> On Thu, Jan 8, 2026 at 3:41 PM Simon Schippers
> >>>>>>>>> <simon.schippers@...dortmund.de> wrote:
> >>>>>>>>>>
> >>>>>>>>>> On 1/8/26 04:38, Jason Wang wrote:
> >>>>>>>>>>> On Thu, Jan 8, 2026 at 5:06 AM Simon Schippers
> >>>>>>>>>>> <simon.schippers@...dortmund.de> wrote:
> >>>>>>>>>>>>
> >>>>>>>>>>>> Introduce {tun,tap}_ring_consume() helpers that wrap __ptr_ring_consume()
> >>>>>>>>>>>> and wake the corresponding netdev subqueue when consuming an entry frees
> >>>>>>>>>>>> space in the underlying ptr_ring.
> >>>>>>>>>>>>
> >>>>>>>>>>>> Stopping of the netdev queue when the ptr_ring is full will be introduced
> >>>>>>>>>>>> in an upcoming commit.
> >>>>>>>>>>>>
> >>>>>>>>>>>> Co-developed-by: Tim Gebauer <tim.gebauer@...dortmund.de>
> >>>>>>>>>>>> Signed-off-by: Tim Gebauer <tim.gebauer@...dortmund.de>
> >>>>>>>>>>>> Signed-off-by: Simon Schippers <simon.schippers@...dortmund.de>
> >>>>>>>>>>>> ---
> >>>>>>>>>>>> drivers/net/tap.c | 23 ++++++++++++++++++++++-
> >>>>>>>>>>>> drivers/net/tun.c | 25 +++++++++++++++++++++++--
> >>>>>>>>>>>> 2 files changed, 45 insertions(+), 3 deletions(-)
> >>>>>>>>>>>>
> >>>>>>>>>>>> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
> >>>>>>>>>>>> index 1197f245e873..2442cf7ac385 100644
> >>>>>>>>>>>> --- a/drivers/net/tap.c
> >>>>>>>>>>>> +++ b/drivers/net/tap.c
> >>>>>>>>>>>> @@ -753,6 +753,27 @@ static ssize_t tap_put_user(struct tap_queue *q,
> >>>>>>>>>>>> return ret ? ret : total;
> >>>>>>>>>>>> }
> >>>>>>>>>>>>
> >>>>>>>>>>>> +static void *tap_ring_consume(struct tap_queue *q)
> >>>>>>>>>>>> +{
> >>>>>>>>>>>> + struct ptr_ring *ring = &q->ring;
> >>>>>>>>>>>> + struct net_device *dev;
> >>>>>>>>>>>> + void *ptr;
> >>>>>>>>>>>> +
> >>>>>>>>>>>> + spin_lock(&ring->consumer_lock);
> >>>>>>>>>>>> +
> >>>>>>>>>>>> + ptr = __ptr_ring_consume(ring);
> >>>>>>>>>>>> + if (unlikely(ptr && __ptr_ring_consume_created_space(ring, 1))) {
> >>>>>>>>>>>> + rcu_read_lock();
> >>>>>>>>>>>> + dev = rcu_dereference(q->tap)->dev;
> >>>>>>>>>>>> + netif_wake_subqueue(dev, q->queue_index);
> >>>>>>>>>>>> + rcu_read_unlock();
> >>>>>>>>>>>> + }
> >>>>>>>>>>>> +
> >>>>>>>>>>>> + spin_unlock(&ring->consumer_lock);
> >>>>>>>>>>>> +
> >>>>>>>>>>>> + return ptr;
> >>>>>>>>>>>> +}
> >>>>>>>>>>>> +
> >>>>>>>>>>>> static ssize_t tap_do_read(struct tap_queue *q,
> >>>>>>>>>>>> struct iov_iter *to,
> >>>>>>>>>>>> int noblock, struct sk_buff *skb)
> >>>>>>>>>>>> @@ -774,7 +795,7 @@ static ssize_t tap_do_read(struct tap_queue *q,
> >>>>>>>>>>>> TASK_INTERRUPTIBLE);
> >>>>>>>>>>>>
> >>>>>>>>>>>> /* Read frames from the queue */
> >>>>>>>>>>>> - skb = ptr_ring_consume(&q->ring);
> >>>>>>>>>>>> + skb = tap_ring_consume(q);
> >>>>>>>>>>>> if (skb)
> >>>>>>>>>>>> break;
> >>>>>>>>>>>> if (noblock) {
> >>>>>>>>>>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> >>>>>>>>>>>> index 8192740357a0..7148f9a844a4 100644
> >>>>>>>>>>>> --- a/drivers/net/tun.c
> >>>>>>>>>>>> +++ b/drivers/net/tun.c
> >>>>>>>>>>>> @@ -2113,13 +2113,34 @@ static ssize_t tun_put_user(struct tun_struct *tun,
> >>>>>>>>>>>> return total;
> >>>>>>>>>>>> }
> >>>>>>>>>>>>
> >>>>>>>>>>>> +static void *tun_ring_consume(struct tun_file *tfile)
> >>>>>>>>>>>> +{
> >>>>>>>>>>>> + struct ptr_ring *ring = &tfile->tx_ring;
> >>>>>>>>>>>> + struct net_device *dev;
> >>>>>>>>>>>> + void *ptr;
> >>>>>>>>>>>> +
> >>>>>>>>>>>> + spin_lock(&ring->consumer_lock);
> >>>>>>>>>>>> +
> >>>>>>>>>>>> + ptr = __ptr_ring_consume(ring);
> >>>>>>>>>>>> + if (unlikely(ptr && __ptr_ring_consume_created_space(ring, 1))) {
> >>>>>>>>>>>
> >>>>>>>>>>> I guess it's the "bug" I mentioned in the previous patch that leads to
> >>>>>>>>>>> the check of __ptr_ring_consume_created_space() here. If it's true,
> >>>>>>>>>>> another call to tweak the current API.
> >>>>>>>>>>>
> >>>>>>>>>>>> + rcu_read_lock();
> >>>>>>>>>>>> + dev = rcu_dereference(tfile->tun)->dev;
> >>>>>>>>>>>> + netif_wake_subqueue(dev, tfile->queue_index);
> >>>>>>>>>>>
> >>>>>>>>>>> This would cause the producer TX_SOFTIRQ to run on the same cpu which
> >>>>>>>>>>> I'm not sure is what we want.
> >>>>>>>>>>
> >>>>>>>>>> What else would you suggest calling to wake the queue?
> >>>>>>>>>
> >>>>>>>>> I don't have a good method in my mind, just want to point out its implications.
> >>>>>>>>
> >>>>>>>> I have to admit I'm a bit stuck at this point, particularly with this
> >>>>>>>> aspect.
> >>>>>>>>
> >>>>>>>> What is the correct way to pass the producer CPU ID to the consumer?
> >>>>>>>> Would it make sense to store smp_processor_id() in the tfile inside
> >>>>>>>> tun_net_xmit(), or should it instead be stored in the skb (similar to the
> >>>>>>>> XDP bit)? In the latter case, my concern is that this information may
> >>>>>>>> already be significantly outdated by the time it is used.
> >>>>>>>>
> >>>>>>>> Based on that, my idea would be for the consumer to wake the producer by
> >>>>>>>> invoking a new function (e.g., tun_wake_queue()) on the producer CPU via
> >>>>>>>> smp_call_function_single().
> >>>>>>>> Is this a reasonable approach?
> >>>>>>>
> >>>>>>> I'm not sure but it would introduce costs like IPI.
> >>>>>>>
> >>>>>>>>
> >>>>>>>> More generally, would triggering TX_SOFTIRQ on the consumer CPU be
> >>>>>>>> considered a deal-breaker for the patch set?
> >>>>>>>
> >>>>>>> It depends on whether or not it has effects on the performance.
> >>>>>>> Especially when vhost is pinned.
> >>>>>>
> >>>>>> I meant we can benchmark to see the impact. For example, pin vhost to
> >>>>>> a specific CPU and the try to see the impact of the TX_SOFTIRQ.
> >>>>>>
> >>>>>> Thanks
> >>>>>>
> >>>>>
> >>>>> I ran benchmarks with vhost pinned to CPU 0 using taskset -p -c 0 ...
> >>>>> for both the stock and patched versions. The benchmarks were run with
> >>>>> the full patch series applied, since testing only patches 1-3 would not
> >>>>> be meaningful - the queue is never stopped in that case, so no
> >>>>> TX_SOFTIRQ is triggered.
> >>>>>
> >>>>> Compared to the non-pinned CPU benchmarks in the cover letter,
> >>>>> performance is lower for pktgen with a single thread but higher with
> >>>>> four threads. The results show no regression for the patched version,
> >>>>> with even slight performance improvements observed:
> >>>>>
> >>>>> +-------------------------+-----------+----------------+
> >>>>> | pktgen benchmarks to | Stock | Patched with |
> >>>>> | Debian VM, i5 6300HQ, | | fq_codel qdisc |
> >>>>> | 100M packets | | |
> >>>>> | vhost pinned to core 0 | | |
> >>>>> +-----------+-------------+-----------+----------------+
> >>>>> | TAP | Transmitted | 452 Kpps | 454 Kpps |
> >>>>> | + +-------------+-----------+----------------+
> >>>>> | vhost-net | Lost | 1154 Kpps | 0 |
> >>>>> +-----------+-------------+-----------+----------------+
> >>>>>
> >>>>> +-------------------------+-----------+----------------+
> >>>>> | pktgen benchmarks to | Stock | Patched with |
> >>>>> | Debian VM, i5 6300HQ, | | fq_codel qdisc |
> >>>>> | 100M packets | | |
> >>>>> | vhost pinned to core 0 | | |
> >>>>> | *4 threads* | | |
> >>>>> +-----------+-------------+-----------+----------------+
> >>>>> | TAP | Transmitted | 71 Kpps | 79 Kpps |
> >>>>> | + +-------------+-----------+----------------+
> >>>>> | vhost-net | Lost | 1527 Kpps | 0 |
> >>>>> +-----------+-------------+-----------+----------------+
> >>>
> >>> The PPS seems to be low. I'd suggest using testpmd (rxonly) mode in
> >>> the guest or an xdp program that did XDP_DROP in the guest.
> >>
> >> I forgot to mention that these PPS values are per thread.
> >> So overall we have 71 Kpps * 4 = 284 Kpps and 79 Kpps * 4 = 326 Kpps,
> >> respectively. For packet loss, that comes out to 1154 Kpps * 4 =
> >> 4616 Kpps and 0, respectively.
> >>
> >> Sorry about that!
> >>
> >> The pktgen benchmarks with a single thread look fine, right?
> >
> > Still looks very low. E.g I just have a run of pktgen (using
> > pktgen_sample03_burst_single_flow.sh) without a XDP_DROP in the guest,
> > I can get 1Mpps.
>
> Keep in mind that I am using an older CPU (i5-6300HQ). For the
> single-threaded tests I always used pktgen_sample01_simple.sh, and for
> the multi-threaded tests I always used pktgen_sample02_multiqueue.sh.
>
> Using pktgen_sample03_burst_single_flow.sh as you did fails for me (even
> though the same parameters work fine for sample01 and sample02):
>
> samples/pktgen/pktgen_sample03_burst_single_flow.sh -i tap0 -m
> 52:54:00:12:34:56 -d 10.0.0.2 -n 100000000
> /samples/pktgen/functions.sh: line 79: echo: write error: Operation not
> supported
> ERROR: Write error(1) occurred
> cmd: "burst 32 > /proc/net/pktgen/tap0@0"
>
> ...and I do not know what I am doing wrong, even after looking at
> Documentation/networking/pktgen.rst. Every burst size except 1 fails.
> Any clues?
Please use -b 0, and I'm Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz.
Another thing I can think of is to disable
1) mitigations in both guest and host
2) any kernel debug features in both host and guest
Thanks
>
> Thanks!
>
> >
> >>
> >> I'll still look into using an XDP program that does XDP_DROP in the
> >> guest.
> >>
> >> Thanks!
> >
> > Thanks
> >
> >>
> >>>
> >>>>>
> >>>>> +------------------------+-------------+----------------+
> >>>>> | iperf3 TCP benchmarks | Stock | Patched with |
> >>>>> | to Debian VM 120s | | fq_codel qdisc |
> >>>>> | vhost pinned to core 0 | | |
> >>>>> +------------------------+-------------+----------------+
> >>>>> | TAP | 22.0 Gbit/s | 22.0 Gbit/s |
> >>>>> | + | | |
> >>>>> | vhost-net | | |
> >>>>> +------------------------+-------------+----------------+
> >>>>>
> >>>>> +---------------------------+-------------+----------------+
> >>>>> | iperf3 TCP benchmarks | Stock | Patched with |
> >>>>> | to Debian VM 120s | | fq_codel qdisc |
> >>>>> | vhost pinned to core 0 | | |
> >>>>> | *4 iperf3 client threads* | | |
> >>>>> +---------------------------+-------------+----------------+
> >>>>> | TAP | 21.4 Gbit/s | 21.5 Gbit/s |
> >>>>> | + | | |
> >>>>> | vhost-net | | |
> >>>>> +---------------------------+-------------+----------------+
> >>>>
> >>>> What are your thoughts on this?
> >>>>
> >>>> Thanks!
> >>>>
> >>>>
> >>>
> >>> Thanks
> >>>
> >>
> >
>
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