[bionic] ConnectX5 Large message size throughput degradation in TCP

Bug #1763325 reported by Talat Batheesh on 2018-04-12
This bug affects 1 person
Affects Status Importance Assigned to Milestone
linux (Ubuntu)

Bug Description

we see degradation ~20% on ConnectX-5/4 in the following case:
TCP, 1 QP, 1 stream, unidir, single port.
Message sizes 1M and up show this degradation.

After changing the default TX moderation mode to off we see up to 40% packet rate and up to 23% bandwidth degradtions.

There is an upstream commit that fix this issue, I will backport it and send it to the <email address hidden>

commit 48bfc39791b8b4a25f165e711f18b9c1617cefbc
Author: Tal Gilboa <email address hidden>
Date: Fri Mar 30 15:50:08 2018 -0700

    net/mlx5e: Set EQE based as default TX interrupt moderation mode

    The default TX moderation mode was mistakenly set to CQE based. The
    intention was to add a control ability in order to improve some specific
    use-cases. In general, we prefer to use EQE based moderation as it gives
    much better numbers for the common cases.

    CQE based causes a degradation in the common case since it resets the
    moderation timer on CQE generation. This causes an issue when TSO is
    well utilized (large TSO sessions). The timer is set to 16us so traffic
    of ~64KB TSO sessions per second would mean timer reset (CQE per TSO
    session -> long time between CQEs). In this case we quickly reach the
    tcp_limit_output_bytes (256KB by default) and cause a halt in TX traffic.

    By setting EQE based moderation we make sure timer would expire after
    16us regardless of the packet rate.
    This fixes an up to 40% packet rate and up to 23% bandwidth degradtions.

    Fixes: 0088cbbc4b66 ("net/mlx5e: Enable CQE based moderation on TX CQ")
    Signed-off-by: Tal Gilboa <email address hidden>
    Signed-off-by: Saeed Mahameed <email address hidden>
    Signed-off-by: David S. Miller <email address hidden>

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
index c71f4f10283b..0aab3afc6885 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
@@ -4137,7 +4137,7 @@ void mlx5e_build_nic_params(struct mlx5_core_dev *mdev,
                            struct mlx5e_params *params,
                            u16 max_channels, u16 mtu)
- u8 cq_period_mode = 0;
+ u8 rx_cq_period_mode;

        params->sw_mtu = mtu;
        params->hard_mtu = MLX5E_ETH_HARD_MTU;
@@ -4173,12 +4173,12 @@ void mlx5e_build_nic_params(struct mlx5_core_dev *mdev,
        params->lro_timeout = mlx5e_choose_lro_timeout(mdev, MLX5E_DEFAULT_LRO_TIMEOUT);

        /* CQ moderation params */
- cq_period_mode = MLX5_CAP_GEN(mdev, cq_period_start_from_cqe) ?
+ rx_cq_period_mode = MLX5_CAP_GEN(mdev, cq_period_start_from_cqe) ?
                        MLX5_CQ_PERIOD_MODE_START_FROM_CQE :
        params->rx_dim_enabled = MLX5_CAP_GEN(mdev, cq_moderation);
- mlx5e_set_rx_cq_mode_params(params, cq_period_mode);
- mlx5e_set_tx_cq_mode_params(params, cq_period_mode);
+ mlx5e_set_rx_cq_mode_params(params, rx_cq_period_mode);
+ mlx5e_set_tx_cq_mode_params(params, MLX5_CQ_PERIOD_MODE_START_FROM_EQE);

        /* TX inline */
        params->tx_min_inline_mode = mlx5e_params_calculate_tx_min_inline(mdev);

Talat Batheesh (talat-b87) wrote :

Testing this patch with bionic and it is working properly

before the patch

# ethtool --show-priv-flags enp6s0f0
Private flags for enp6s0f0:
rx_cqe_moder : on
tx_cqe_moder : on
rx_cqe_compress: off

After applying the patch

# ethtool --show-priv-flags enp6s0f0
Private flags for enp6s0f0:
rx_cqe_moder : on
tx_cqe_moder : off
rx_cqe_compress: off

This bug is missing log files that will aid in diagnosing the problem. While running an Ubuntu kernel (not a mainline or third-party kernel) please enter the following command in a terminal window:

apport-collect 1763325

and then change the status of the bug to 'Confirmed'.

If, due to the nature of the issue you have encountered, you are unable to run this command, please add a comment stating that fact and change the bug status to 'Confirmed'.

This change has been made by an automated script, maintained by the Ubuntu Kernel Team.

Changed in linux (Ubuntu):
status: New → Incomplete
tags: added: bionic
Seth Forshee (sforshee) wrote :

The fix was included in the 4.15.17 stable update. Duping this bug to that one.

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