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Message-ID: <197573a1-df52-4928-adb9-55a7a4f78839@tu-dortmund.de>
Date: Wed, 4 Feb 2026 16:43:56 +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 2/3/26 04:48, Jason Wang wrote:
> On Mon, Feb 2, 2026 at 4:19 AM Simon Schippers
> <simon.schippers@...dortmund.de> wrote:
>>
>> On 1/30/26 02:51, Jason Wang wrote:
>>> 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.
>>
>> I tried using "-b 0", and while it worked, there was no noticeable
>> performance improvement.
>>
>>>
>>> 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
>>
>> I also rebuilt the kernel with everything disabled under
>> "Kernel hacking", but that didn’t make any difference either.
>>
>> Because of this, I ran "pktgen_sample01_simple.sh" and
>> "pktgen_sample02_multiqueue.sh" on my AMD Ryzen 5 5600X system. The
>> results were about 374 Kpps with TAP and 1192 Kpps with TAP+vhost_net,
>> with very similar performance between the stock and patched kernels.
>>
>> Personally, I think the low performance is to blame on the hardware.
>
> Let's double confirm this by:
>
> 1) make sure pktgen is using 100% CPU
> 2) Perf doesn't show anything strange for pktgen thread
>
> Thanks
>
I ran pktgen using pktgen_sample01_simple.sh and, in parallel, started a
100 second perf stat measurement covering all kpktgend threads.
Across all configurations, a single CPU was fully utilized.
Apart from that, the patched variants show a higher branch frequency and
a slightly increased number of context switches.
The detailed results are provided below:
Processor: Ryzen 5 5600X
pktgen command:
sudo perf stat samples/pktgen/pktgen_sample01_simple.sh -i tap0 -m
52:54:00:12:34:56 -d 10.0.0.2 -n 10000000000
perf stat command:
sudo perf stat --timeout 100000 -p $(pgrep kpktgend | tr '\n' ,) -o X.txt
Results:
Stock TAP:
46.997 context-switches # 467,2 cs/sec cs_per_second
0 cpu-migrations # 0,0 migrations/sec migrations_per_second
0 page-faults # 0,0 faults/sec page_faults_per_second
100.587,69 msec task-clock # 1,0 CPUs CPUs_utilized
8.491.586.483 branch-misses # 10,9 % branch_miss_rate (50,24%)
77.734.761.406 branches # 772,8 M/sec branch_frequency (66,85%)
382.420.291.585 cpu-cycles # 3,8 GHz cycles_frequency (66,85%)
377.612.185.141 instructions # 1,0 instructions insn_per_cycle (66,85%)
84.012.185.936 stalled-cycles-frontend # 0,22 frontend_cycles_idle (66,35%)
100,100414494 seconds time elapsed
Stock TAP+vhost-net:
47.087 context-switches # 468,1 cs/sec cs_per_second
0 cpu-migrations # 0,0 migrations/sec migrations_per_second
0 page-faults # 0,0 faults/sec page_faults_per_second
100.594,09 msec task-clock # 1,0 CPUs CPUs_utilized
8.034.703.613 branch-misses # 11,1 % branch_miss_rate (50,24%)
72.477.989.922 branches # 720,5 M/sec branch_frequency (66,86%)
382.218.276.832 cpu-cycles # 3,8 GHz cycles_frequency (66,85%)
349.555.577.281 instructions # 0,9 instructions insn_per_cycle (66,85%)
83.917.644.262 stalled-cycles-frontend # 0,22 frontend_cycles_idle (66,35%)
100,100520402 seconds time elapsed
Patched TAP:
47.862 context-switches # 475,8 cs/sec cs_per_second
0 cpu-migrations # 0,0 migrations/sec migrations_per_second
0 page-faults # 0,0 faults/sec page_faults_per_second
100.589,30 msec task-clock # 1,0 CPUs CPUs_utilized
9.337.258.794 branch-misses # 9,4 % branch_miss_rate (50,19%)
99.518.421.676 branches # 989,4 M/sec branch_frequency (66,85%)
382.508.244.894 cpu-cycles # 3,8 GHz cycles_frequency (66,85%)
312.582.270.975 instructions # 0,8 instructions insn_per_cycle (66,85%)
76.338.503.984 stalled-cycles-frontend # 0,20 frontend_cycles_idle (66,39%)
100,101262454 seconds time elapsed
Patched TAP+vhost-net:
47.892 context-switches # 476,1 cs/sec cs_per_second
0 cpu-migrations # 0,0 migrations/sec migrations_per_second
0 page-faults # 0,0 faults/sec page_faults_per_second
100.581,95 msec task-clock # 1,0 CPUs CPUs_utilized
9.083.588.313 branch-misses # 10,1 % branch_miss_rate (50,28%)
90.300.124.712 branches # 897,8 M/sec branch_frequency (66,85%)
382.374.510.376 cpu-cycles # 3,8 GHz cycles_frequency (66,85%)
340.089.181.199 instructions # 0,9 instructions insn_per_cycle (66,85%)
78.151.408.955 stalled-cycles-frontend # 0,20 frontend_cycles_idle (66,31%)
100,101212911 seconds time elapsed
>>
>> Thanks!
>>
>>>
>>> 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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