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

[Yangchao Zhou <zhouyates@gmail.com>]

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.

Fixes: 3e12a98fe397 ("kni: optimize Rx burst")
Cc: stable@dpdk.org

Signed-off-by: Yangchao Zhou <zhouyates@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;
-- 
2.25.1