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

[Hengqi Chen]

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.
>

Thanks for the testing. Will investigate.

> 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
>