[PATCH v3] kni: fix possible alloc_q starvation when mbufs are exhausted

[Matt]

Hi Ferruh,

In my case, the traffic is not large, so I can't see the impact.
I also tested under high load(>2Mpps with 2 DPDK cores and 2 kernel threads)
and found no significant difference in performance either.
I think the reason should be that it will not
run to 'kni_fifo_count(kni->alloc_q) == 0' under high load.

On Tue, Jan 3, 2023 at 8:47 PM Ferruh Yigit <ferruh.yigit at amd.com> wrote:

> On 12/30/2022 4:23 AM, Yangchao Zhou wrote:
> > In some scenarios, mbufs returned by rte_kni_rx_burst are not freed
> > immediately. So kni_allocate_mbufs may be failed, but we don't know.
> >
> > Even worse, when alloc_q is completely exhausted, kni_net_tx in
> > rte_kni.ko will drop all tx packets. kni_allocate_mbufs is never
> > called again, even if the mbufs are eventually freed.
> >
> > In this patch, we try to allocate mbufs for alloc_q when it is empty.
> >
> > According to historical experience, the performance bottleneck of KNI
> > is offen the usleep_range of kni thread in rte_kni.ko.
> > The check of kni_fifo_count is trivial and the cost should be acceptable.
> >
>
> Hi Yangchao,
>
> Are you observing any performance impact with this change in you use case?
>
>
> > Fixes: 3e12a98fe397 ("kni: optimize Rx burst")
> > Cc: stable at dpdk.org
> >
> > Signed-off-by: Yangchao Zhou <zhouyates at gmail.com>
> > ---
> >  lib/kni/rte_kni.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/lib/kni/rte_kni.c b/lib/kni/rte_kni.c
> > index 8ab6c47153..bfa6a001ff 100644
> > --- a/lib/kni/rte_kni.c
> > +++ b/lib/kni/rte_kni.c
> > @@ -634,8 +634,8 @@ rte_kni_rx_burst(struct rte_kni *kni, struct
> rte_mbuf **mbufs, unsigned int num)
> >  {
> >       unsigned int ret = kni_fifo_get(kni->tx_q, (void **)mbufs, num);
> >
> > -     /* If buffers removed, allocate mbufs and then put them into
> alloc_q */
> > -     if (ret)
> > +     /* If buffers removed or alloc_q is empty, allocate mbufs and then
> put them into alloc_q */
> > +     if (ret || (kni_fifo_count(kni->alloc_q) == 0))
> >               kni_allocate_mbufs(kni);
> >
> >       return ret;
>
>
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