On Thu, Jan 29, 2026 at 5:25 PM Simon Schippers <[email protected]> wrote: > > On 1/29/26 02:14, Jason Wang wrote: > > On Wed, Jan 28, 2026 at 3:54 PM Simon Schippers > > <[email protected]> wrote: > >> > >> On 1/28/26 08:03, Jason Wang wrote: > >>> 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. > >> > >> 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. 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 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 > >>> > >> > > >
