| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <CACGkMEsVMz+vS3KxykYBGXvyt3MZcstnYWUiYJZhLSMoHC5Smw@mail.gmail.com>
Date: Thu, 29 Jan 2026 09:14:38 +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 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.
>
> 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
> >
>
Powered by blists - more mailing lists