On Wed, Jan 28, 2026 at 12:48 AM Simon Schippers <[email protected]> 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 <[email protected]> wrote: > >>> > >>> On Wed, Jan 21, 2026 at 5:33 PM Simon Schippers > >>> <[email protected]> wrote: > >>>> > >>>> On 1/9/26 07:02, Jason Wang wrote: > >>>>> On Thu, Jan 8, 2026 at 3:41 PM Simon Schippers > >>>>> <[email protected]> wrote: > >>>>>> > >>>>>> On 1/8/26 04:38, Jason Wang wrote: > >>>>>>> On Thu, Jan 8, 2026 at 5:06 AM Simon Schippers > >>>>>>> <[email protected]> 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 <[email protected]> > >>>>>>>> Signed-off-by: Tim Gebauer <[email protected]> > >>>>>>>> Signed-off-by: Simon Schippers <[email protected]> > >>>>>>>> --- > >>>>>>>> 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. > > > > +------------------------+-------------+----------------+ > > | 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
