[dpdk-dev] [Bug 627] mlx5 / DPDK 20.11: severe RSS misbehavior and performance drop with libibverbs 29

[dpdk-dev] [Bug 627] mlx5 / DPDK 20.11: severe RSS misbehavior and performance drop with libibverbs 29

[bugzilla]

https://bugs.dpdk.org/show_bug.cgi?id=627

            Bug ID: 627
           Summary: mlx5 / DPDK 20.11: severe RSS misbehavior and
                    performance drop with libibverbs 29
           Product: DPDK
           Version: 20.11
          Hardware: x86
                OS: Linux
            Status: UNCONFIRMED
          Severity: major
          Priority: Normal
         Component: ethdev
          Assignee: dev@dpdk.org
          Reporter: martin.weiser@allegro-packets.com
  Target Milestone: ---

When updating to DPDK v20.11 we noticed a dramatic performance drop and RSS
misbehavior with our ConnectX-5 NICs.
Some of the rx queues do not receive any traffic while others only receive a
fraction of what they should receive.
Overall the receive performance dropped by around 90%.

We then were able to identify that this only happens when the default
mlx5_rx_burst path is used and the issue
does not appear when a vectorized rx function is used. We are forced to the
default path since we use multi-segment
mbufs to handle jumbo frames which automatically drops you to the default
non-vectorized path in mlx5.

After digging a bit deeper we could identify the following commit to cause the
issue:

54c2d46b160f8ad0bff0977812bf871ca5dd8241: net/mlx5: support flow tag and packet
header miniCQEs

Reverting this commit restored the original performance.
But on a test system running testpmd we were not able to replicate this
behavior. It took us quite some time to
identify the reason for the difference:

On the test system we have an installation of libibverbs 33 and this version
has the symbol
mlx5dv_dr_action_create_dest_devx_tir in infiniband/mlx5dv.h. This causes the
meson build to set
HAVE_MLX5DV_DR_ACTION_DEST_DEVX_TIR during compilation which in turn causes
config->dest_tir to be set in the
driver. It seems that the whole driver behaves quite differently if this flag
is set.
On our regular system we have an installation of libibverbs 29 which lacks the
aforementioned symbol and if we
compile testpmd on that system it also shows the misbehavior. But only if you
force it to use the
non-vectorized mlx5_rx_burst path.

We used the following testpmd command line to replicate the behavior:

`./app/dpdk-testpmd -n 4 --legacy-mem -w 81:00.0 -w 81:00.1 -l 1,2,3,4,5 --
--total-num-mbufs=2000000 --nb-cores=4 --rxq=4 --txq=4 --max-pkt-len=15360
--rx-offloads=0x12800 --mbuf-size=2331 --rxd=4096`

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