[dpdk-users] IPV4 and VLAN RX packets from NIC donot have the mbuf offload flag set with DPDK 18.08 version

Van Haaren, Harry harry.van.haaren at intel.com
Thu Jun 27 18:43:03 CEST 2019


Hi,

> -----Original Message-----
> From: users [mailto:users-bounces at dpdk.org] On Behalf Of Suchetha p
> Sent: Tuesday, June 25, 2019 3:14 PM
> To: users at dpdk.org
> Subject: [dpdk-users] IPV4 and VLAN RX packets from NIC donot have the mbuf
> offload flag set with DPDK 18.08 version
> 
> Hi,
> 
> 
> After upgrading the DPDK to 18.08 version and RHEL 7.6 OS I observe that
> offload flags were not set in the packets received from NIC for eth type
> VLAN and IPV4.
> 
> Hence application had to explicitly set the offload flags for VLAN and IPV4
> packet.
> 
> if (ETHER_TYPE_VLAN == eth_type) {
>           m->ol_flags |= PKT_RX_VLAN_PKT;
> 
>     }
>     if (ETHER_TYPE_IPv4 == eth_type) {
>           m->ol_flags |= PKT_RX_IPV4_HDR;
>          }

Do you expect the issue described below is a performance issue from the above code?

It seems quite "branchy", meaning branch-misses might have a high
performance impact if your incoming traffic is not predictable.

From a code point of view, the following achieves the same effect, but without
any branching on the datapath;

m->ol_flags |= (ETHER_TYPE_VLAN == eth_type) << PKT_RX_VLAN_PKT_SHIFT;

Notice that the comparison (ETHER_TYPE_VLAN == eth_type) will result in a 1 or 0,
and using |= with a 0 value won't change the m->ol_flags bit contents.


If the performance impact of branch-mispredicts is the root cause for your
issue, try the above code and see does it help/solve.


> If these offload flags were not set in the mbuf the packet would get
> dropped and will not be forwarded to the Tap device.
> 
> But the offload flag setting per NIC received packet is causing some
> performance issue and delay in data transfer.
> 
> TCP segment loss and retransmissions are observed and TX Tap drops are
> observed.
> 
> These changes with respect to offload flag setting for RX VLAN and IPV4
> packet was not required with old DPDK and RHEL6 and packets were getting
> forwarded from NIC to Tap device and vice versa without any issue.
> 
> So is anything changed with respect to DPDK and RHEL 7 for ingress packets ?

I'll let others reply here - I don't know, sorry.

<snip>

Hope that helps, -Harry


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