On Wed, Jan 28, 2026 at 12:48 AM Simon Schippers
[off-list ref] wrote:

On 1/23/26 10:54, Simon Schippers wrote:

quoted

On 1/23/26 04:05, Jason Wang wrote:

quoted

On Thu, Jan 22, 2026 at 1:35 PM Jason Wang [off-list ref] wrote:

quoted

On Wed, Jan 21, 2026 at 5:33 PM Simon Schippers
[off-list ref] wrote:

quoted

On 1/9/26 07:02, Jason Wang wrote:

quoted

On Thu, Jan 8, 2026 at 3:41 PM Simon Schippers
[off-list ref] wrote:

quoted

On 1/8/26 04:38, Jason Wang wrote:

quoted

On Thu, Jan 8, 2026 at 5:06 AM Simon Schippers
[off-list ref] wrote:

quoted

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 <redacted>
Signed-off-by: Tim Gebauer <redacted>
Signed-off-by: Simon Schippers <redacted>
---
 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.

quoted

+               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.

quoted

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.

quoted

+------------------------+-------------+----------------+
| 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