xsk stats report Tx_ring_empty_descs when change argument of GetDescs()

[seudut]

hi,

I encounter a issue when test this library. Based on the example sendudp.go, I just make two change

• change the default option (change the size of NufFrames, default is 128, I change it to 16) when create xsk options
• change the argument when call GetDescs(), the default in sendudp.go is get number of free tx slots, for test, I change the number as 1, just to get one free descrs. here is the detail change of sendudp.go

@@ -51,7 +52,16 @@ func main() {
                panic(err)
        }

-       xsk, err := xdp.NewSocket(link.Attrs().Index, QueueID, nil)
+       options := xdp.SocketOptions{
+               NumFrames:              128,
+               FrameSize:              2048,
+               FillRingNumDescs:       2048,
+               CompletionRingNumDescs: 2048,
+               RxRingNumDescs:         2048,
+               TxRingNumDescs:         2048,
+       }
+
+       xsk, err := xdp.NewSocket(link.Attrs().Index, QueueID, &options)
        if err != nil {
                panic(err)
        }
@@ -122,12 +132,14 @@ func main() {
                        }
                        numPkts = cur.Completed - prev.Completed
                        fmt.Printf("%d packets/s (%d Mb/s)\n", numPkts, (numPkts*uint64(frameLen)*8)/(1000*1000))
+                       ss, _ := json.Marshal(cur)
+                       fmt.Printf("stats:%s\n", string(ss))
                        prev = cur
                }
        }()

        for {
-               descs := xsk.GetDescs(xsk.NumFreeTxSlots())
+               descs := xsk.GetDescs(1)
                for i := range descs {
                        descs[i].Len = uint32(frameLen)
                }
@@ -137,5 +149,6 @@ func main() {
                if err != nil {
                        panic(err)
                }
+               time.Sleep(1000 * time.Millisecond)
        }
 }

the demo still be able to work, but when print the stats, Tx_ring_empty_descs increase as the packet send out, here is the log,

0 packets/s (0 Mb/s)
stats:{"Filled":0,"Received":0,"Transmitted":1,"Completed":0,"KernelStats":{"Rx_dropped":0,"Rx_invalid_descs":0,"Tx_invalid_descs":0,"Rx_ring_full":0,"Rx_fill_ring_empty_descs":0,"Tx_ring_empty_descs":1}}
2 packets/s (0 Mb/s)
stats:{"Filled":0,"Received":0,"Transmitted":3,"Completed":2,"KernelStats":{"Rx_dropped":0,"Rx_invalid_descs":0,"Tx_invalid_descs":0,"Rx_ring_full":0,"Rx_fill_ring_empty_descs":0,"Tx_ring_empty_descs":3}}
1 packets/s (0 Mb/s)
stats:{"Filled":0,"Received":0,"Transmitted":4,"Completed":3,"KernelStats":{"Rx_dropped":0,"Rx_invalid_descs":0,"Tx_invalid_descs":0,"Rx_ring_full":0,"Rx_fill_ring_empty_descs":0,"Tx_ring_empty_descs":4}}
1 packets/s (0 Mb/s)
stats:{"Filled":0,"Received":0,"Transmitted":5,"Completed":4,"KernelStats":{"Rx_dropped":0,"Rx_invalid_descs":0,"Tx_invalid_descs":0,"Rx_ring_full":0,"Rx_fill_ring_empty_descs":0,"Tx_ring_empty_descs":5}}
1 packets/s (0 Mb/s)
stats:{"Filled":0,"Received":0,"Transmitted":6,"Completed":5,"KernelStats":{"Rx_dropped":0,"Rx_invalid_descs":0,"Tx_invalid_descs":0,"Rx_ring_full":0,"Rx_fill_ring_empty_descs":0,"Tx_ring_empty_descs":6}}

check the linux document https://lore.kernel.org/bpf/20200708072835.4427-2-ciara.loftus@intel.com/
Tx_ring_empty_descs means failed to retrieve descriptor from tx ring.

Is this an issue ? and in what situation, will this happen?