| 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: <3a1d6232-efe4-4e79-a196-44794fdc0f33@tu-dortmund.de>
Date: Tue, 27 Jan 2026 17:47:38 +0100
From: Simon Schippers <simon.schippers@...dortmund.de>
To: Jason Wang <jasowang@...hat.com>
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: [PATCH net-next v7 3/9] tun/tap: add ptr_ring consume helper with
netdev queue wakeup
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 |
> +-----------+-------------+-----------+----------------+
>
> +------------------------+-------------+----------------+
> | 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!
Powered by blists - more mailing lists