[dpdk-dev] Sync up status for Mellanox PMD barrier investigation

[Phil Yang (Arm Technology China)]

Some update for this thread.

In the most critical datapath of mlx5 PMD, there are some rte_cio_w/rmb, 'dmb osh' on aarch64, in use.
C11 atomic is good for replacing the rte_smp_r/wmb to relax the data synchronization barrier between CPUs.
However, mlx5 PMD needs to write data back to the  HW, so it used a lot of rte_cio_r/wmb to synchronize data.

Please check details below. All comments are welcomed. Thanks.

//////////////////// Data path ///////////////////////////
drivers/net/mlx5/mlx5_rxtx.c=950=mlx5_rx_err_handle(struct mlx5_rxq_data *rxq, uint8_t mbuf_prepare)
drivers/net/mlx5/mlx5_rxtx.c:1002:           rte_cio_wmb();
drivers/net/mlx5/mlx5_rxtx.c:1004:           rte_cio_wmb();
drivers/net/mlx5/mlx5_rxtx.c:1010:           rte_cio_wmb();
drivers/net/mlx5/mlx5_rxtx.c=1272=mlx5_rx_burst(void *dpdk_rxq, struct rte_mbuf **pkts, uint16_t pkts_n)
drivers/net/mlx5/mlx5_rxtx.c:1385:   rte_cio_wmb();
drivers/net/mlx5/mlx5_rxtx.c:1387:   rte_cio_wmb();
drivers/net/mlx5/mlx5_rxtx.c=1549=mlx5_rx_burst_mprq(void *dpdk_rxq, struct rte_mbuf **pkts, uint16_t pkts_n)
drivers/net/mlx5/mlx5_rxtx.c:1741:   rte_cio_wmb();
drivers/net/mlx5/mlx5_rxtx.c:1745:       rte_cio_wmb();
drivers/net/mlx5/mlx5_rxtx_vec_neon.h=366=rxq_burst_v(struct mlx5_rxq_data *rxq, struct rte_mbuf **pkts, uint16_t pkts_n,
drivers/net/mlx5/mlx5_rxtx_vec_neon.h:530:       rte_cio_rmb();

Commit messages:
net/mlx5: cleanup memory barriers: mlx5_rx_burst
https://git.dpdk.org/dpdk/commit/?id=9afa3f74658afc0e21fbe5c3884c55a21ff49299

net/mlx5: add Multi-Packet Rx support : mlx5_rx_burst_mprq
https://git.dpdk.org/dpdk/commit/?id=7d6bf6b866b8c25ec06539b3eeed1db4f785577c

net/mlx5: use coherent I/O memory barrier
https://git.dpdk.org/dpdk/commit/drivers/net/mlx5/mlx5_rxtx.c?id=0cfdc1808de82357a924a479dc3f89de88cd91c2

net/mlx5: extend Rx completion with error handling
https://git.dpdk.org/dpdk/commit/drivers/net/mlx5/mlx5_rxtx.c?id=88c0733535d6a7ce79045d4d57a1d78d904067c8

net/mlx5: fix synchronization on polling Rx completions
https://git.dpdk.org/dpdk/commit/?id=1742c2d9fab07e66209f2d14e7daa50829fc4423


Thanks,
Phil Yang

From: Phil Yang (Arm Technology China)
Sent: Thursday, August 15, 2019 6:35 PM
To: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>
Subject: Sync up status for Mellanox PMD barrier investigation

Hi Honnappa,

I have checked all the barriers in mlx5 PMD data path. In my understanding, it used the barrier correctly (Use DMB to synchronize the memory data between CPUs).
The attachment is the list of positions of these barriers.
I just want to sync up with you the status. Do you have any idea or suggestion on which part should we start to optimization?

Best Regards,
Phil Yang