[dpdk-users] occasionally traffic stalls due to rx and tx descriptor not available

Amarnath Nallapothula Amarnath.Nallapothula at riverbed.com
Fri Jul 6 13:49:40 CEST 2018

I debugged further by attaching my process to gdb and as I suspected transmission is failing due to no free descriptor available and code is unable to free as well due to following condition in fm10k driver.

static inline int __attribute__((always_inline))
fm10k_tx_free_bufs(struct fm10k_tx_queue *txq)
        struct rte_mbuf **txep;
        uint8_t flags;
        uint32_t n;
        uint32_t i;
        int nb_free = 0;
        struct rte_mbuf *m, *free[RTE_FM10K_TX_MAX_FREE_BUF_SZ];

        /* check DD bit on threshold descriptor */
        flags = txq->hw_ring[txq->next_dd].flags;
        if (!(flags & FM10K_TXD_FLAG_DONE))
                return 0; <== returns from here.

Breakpoint 5, fm10k_xmit_pkts_vec (tx_queue=0x7fde3e430040, tx_pkts=0x7fde913eda40, nb_pkts=32)
    at /src/dpdk/drivers/net/fm10k/fm10k_rxtx_vec.c:826
826     in /src/dpdk/drivers/net/fm10k/fm10k_rxtx_vec.c
(gdb) p *(struct fm10k_tx_queue *)tx_queue
$19 = {sw_ring = 0x7fde3e42f000, hw_ring = 0x7fde3e3aef80, hw_ring_phys_addr = 14490988416,
  rs_tracker = {list = 0x7fde3e3aef00, head = 0x0, tail = 0x0, endp = 0x0},
  ops = 0x8adec8 <vec_txq_ops>, last_free = 0, next_free = 191, nb_free = 0, nb_used = 0,
  free_thresh = 32, rs_thresh = 32, next_rs = 191, next_dd = 223, tail_ptr = 0x7fde52020014,
  txq_flags = 3841, nb_desc = 512, port_id = 0 '\000', tx_deferred_start = 0 '\000', queue_id = 0,
  tx_ftag_en = 0}
(gdb) p /x ((struct fm10k_tx_queue *)tx_queue)->hw_ring[223].flags
$21 = 0x60
(gdb) p 0x80 & ((struct fm10k_tx_queue *)tx_queue)->hw_ring[223].flags
$22 = 0

Looks like driver/NIC is unable to transmit packet and hence flags is still not set to FM10K_TXD_FLAG_DONE. But I am still not sure where is the problem.


From: Hui Liu <huiliu0213 at gmail.com>
Date: Friday, 6 July 2018 at 9:06 AM
To: Amarnath Nallapothula <Amarnath.Nallapothula at riverbed.com>
Cc: "users at dpdk.org" <users at dpdk.org>
Subject: Re: [dpdk-users] occasionally traffic stalls due to rx and tx descriptor not available

Hi Amar,

I'm a DPDK newbie and I saw a similar problem recently on one 82599 port. My app is doing a job like this:
1. TX thread calls rte_pktmbuf_alloc() to allocate buffers from mbuf_pool and fills it as ICMP packet and sends out, with speed of around 400,000 packets/sec, 1.6Gbps;
2. RX thread receives ICMP responses and worker threads work with the responses.

This app was running fine for some time, typically from 8 hours to 5 days randomly, then it goes into a bad state, that TX thread could not send packets out any more via rte_eth_tx_buffer() or rte_eth_tx_buffer_flush() while rte_eth_tx_buffer_count_callback() is called for all packets flush. I'm highly suspecting the problem with descriptor exhausted but not get it clear yet..

In my app, I set max pkt burst as 256, rx descriptor as 2048, tx descriptor as 4096 with single rx/tx queue for one port to get good performance, not sure if they are the best combination. Just FYI. For descriptor problem, I'm still investigating on what kind of behavior/condition takes descriptors and never release it, just as your Query 2. If applicable, would you please let me know if there is a way to get the number of available tx/rx descriptor of ports and I could see when descriptors are really taken without being released time by time?

Due to my system environment limit, I'm not able to directly attach gdb to debug... While I'm investigating this problem, would you please update me when you have any clue on your issue and I might get some inspiration from you?

Thank you very much!


On Thu, Jul 5, 2018 at 4:34 AM, Amarnath Nallapothula <Amarnath.Nallapothula at riverbed.com<mailto:Amarnath.Nallapothula at riverbed.com>> wrote:
Hi Experts,

I am testing performance of my dpdk based application which forwards packets from port 1 to port 2 of 40G NIC card and via versa.Occasionally we see that packets rx and tx stops on one of the port. I looked through the dpdk’s fm10k driver’s code and found out that this could happen if rx/tx descriptors are not available.

To improve performance, I am using RSS functionality and created five rx and tx queue. Dedicated lcores are assigned to forward packets from port1 queue 0 to port2 queue 0 and via versa.

During port initialization rx_queue is initialized with 128 Rx ring descriptor size and tx_queue  is initialized 512 Tx ring descriptor. Threshold values are left default.

I have few queries here:

  1.  Is above initialization value for rx and tx descriptor is good for each queue for given port.
  2.  Under what conditions rx and tx descriptor gets exhausted?
  3.  Any suggestion or information you can provide to debug this issue?


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