On 1/30/26 02:51, Jason Wang wrote: > 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.
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. 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 >>>>> >>>> >>> >> >
