[PATCH] net/virtio: Fix check of threshold for Tx freeing

[Jiang, YuX]

Hi,

cd /opt/trex/v3.00/;./t-rex-64 -i --cfg /etc/trex_cfg.yaml -c 16
cp stl/bench.py stl/jy-bench.py
cd /opt/trex/v3.00/;./trex-console
start -f stl/jy-bench.py --port 0 -m 100% -t vm=random

stl/jy-bench.py modify base_pkt.
    def create_stream (self, stl_flow, size, vm, src, dst, pps = 1, isg = 0):
        # Create base packet and pad it to size
        #base_pkt = Ether()/IP(src=src, dst=dst)/UDP(dport=12,sport=1025,chksum=0)
        base_pkt = Ether(dst="40:a6:b7:96:e4:00")/IP(src="192.168.4.1")/("X"*1480)  

Best regards,
Yu Jiang

> -----Original Message-----
> From: Hengqi Chen <hengqi.chen at gmail.com>
> Sent: Thursday, July 17, 2025 11:04 PM
> To: Jiang, YuX <yux.jiang at intel.com>
> Cc: dev at dpdk.org; Baoyuan Li <updoing at sina.com>; Maxime Coquelin
> <maxime.coquelin at redhat.com>; Chenbo Xia <chenbox at nvidia.com>
> Subject: Re: [PATCH] net/virtio: Fix check of threshold for Tx freeing
> 
> Hi Yu,
> 
> On Fri, Jul 4, 2025 at 7:29 PM Jiang, YuX <yux.jiang at intel.com> wrote:
> >
> > Hi Hengqi,
> >
> > Can you help to check the ticket:
> https://bugs.dpdk.org/show_bug.cgi?id=1747? The perf drop is caused by
> your patch. Thanks so much.
> >
> 
> Could you please elaborate how to use trex in your reproduce steps ? Thanks.
> 
> Cheers,
> ---
> Hengqi
> 
> > Best regards,
> > Yu Jiang
> >
> > > -----Original Message-----
> > > From: Hengqi Chen <hengqi.chen at gmail.com>
> > > Sent: Monday, June 9, 2025 3:24 PM
> > > To: dev at dpdk.org
> > > Cc: Hengqi Chen <hengqi.chen at gmail.com>; Baoyuan Li
> > > <updoing at sina.com>; Maxime Coquelin <maxime.coquelin at redhat.com>;
> > > Chenbo Xia <chenbox at nvidia.com>
> > > Subject: [PATCH] net/virtio: Fix check of threshold for Tx freeing
> > >
> > > Like most dirvers, make the fast path of virtio_xmit_cleanup()
> > > behave as described by the comments of rte_eth_txconf::tx_free_thresh
> ([0]):
> > >     Start freeing Tx buffers if there are
> > >     less free descriptors than this value.
> > >
> > > The rationale behind this change is that:
> > >   * vq->vq_nentries is set during device probe with the queue size specified
> > >     by vhost backend, this value does not reflect the real nb_tx_desc
> > >   * the real available tx desc is set to vq->vq_free_cnt via the nb_tx_desc
> > >     param of rte_eth_tx_queue_setup() API
> > >   * so `nb_used > vq->vq_nentries - vq->vq_free_thresh` could never be
> true
> > >     if say nb_tx_desc=2048, vq->vq_nentries=4096, vq->vq_free_thresh=32,
> > >     see bug report 1716 ([1]) for details.
> > >
> > > Bugzilla ID: 1716
> > >
> > >   [0]: https://github.com/DPDK/dpdk/commit/72514b5d5543
> > >   [1]: https://bugs.dpdk.org/show_bug.cgi?id=1716
> > >
> > > Signed-off-by: Baoyuan Li <updoing at sina.com>
> > > Signed-off-by: Hengqi Chen <hengqi.chen at gmail.com>
> > > ---
> > >  drivers/net/virtio/virtio_rxtx.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/net/virtio/virtio_rxtx.c
> > > b/drivers/net/virtio/virtio_rxtx.c
> > > index edecd2011f..ab97f03d7d 100644
> > > --- a/drivers/net/virtio/virtio_rxtx.c
> > > +++ b/drivers/net/virtio/virtio_rxtx.c
> > > @@ -1873,7 +1873,7 @@ virtio_xmit_pkts(void *tx_queue, struct
> > > rte_mbuf **tx_pkts, uint16_t nb_pkts)
> > >
> > >       nb_used = virtqueue_nused(vq);
> > >
> > > -     if (likely(nb_used > vq->vq_nentries - vq->vq_free_thresh))
> > > +     if (likely(vq->vq_free_cnt < vq->vq_free_thresh))
> > >               virtio_xmit_cleanup(vq, nb_used);
> > >
> > >       for (nb_tx = 0; nb_tx < nb_pkts; nb_tx++) {
> > > --
> > > 2.43.5
> >