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

[Hengqi Chen]

Hi Maxime and other maintainers,

On Fri, Jul 4, 2025 at 7:57 PM Maxime Coquelin
<maxime.coquelin at redhat.com> wrote:
>
> Hi Yu, Hengqi,
>
> First, thanks Yu for testing and reporting the performance degradation,
> this is much appreciated!
>
> On 7/4/25 1:29 PM, Jiang, YuX 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.
>
> The options I see are:
> 1. Revert the patch for v25.07, fix it and resubmit for v25.11.
> 2. Fix it on time for v25.07-rc3
> 3. Keep the patch in v25.07 and try to fix it in v25.11.
>
> As I will be OoO starting this evening, I would be in favor of option 1.
> What do you think?
>

I am OK with both option 1 and 3.
Please handle it according to the DPDK community rules.

Thanks,
Hengqi

> Thanks,
> Maxime
>
> > 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
> >
>