Looking at timestamped output of netdev CI reveals that
most of the time in forwarding tests for custom route
hashing is spent on a single case, namely the test which
uses ping (mausezahn does not support flow labels).

On a non-debug kernel we spend 714 of 730 total test
runtime (97%) on this test case. While having flow label
support in a traffic gen tool / mausezahn would be best,
we can significantly speed up the loop by putting ip vrf exec
outside of the iteration.

In a test of 1000 pings using a normal loop takes 50 seconds
to finish. While using:

  ip vrf exec $vrf sh -c "$loop-body"

takes 12 seconds (1/4 of the time).

Some of the slowness is likely due to our inefficient virtualization
setup, but even on my laptop running "ip link help" 16k times takes
25-30 seconds, so I think it's worth optimizing even for fastest
setups.
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20240817203659.712085-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>