selftests: tc-testing: cap parallel tdc to 4 cores

We have observed a lot of lock contention and test instability when running with >8 cores.
Enough to actually make the tests run slower than with fewer cores.

Cap the maximum cores of parallel tdc to 4 which showed in testing to
be a reasonable number for efficiency and stability in different kernel
config scenarios.
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://lore.kernel.org/r/20231117171208.2066136-2-pctammela@mojatatu.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/tc-testing/tdc.py

@@ -1017,6 +1017,7 @@ def main():
     parser = pm.call_add_args(parser)
     (args, remaining) = parser.parse_known_args()
     args.NAMES = NAMES
+    args.mp = min(args.mp, 4)
     pm.set_args(args)
     check_default_settings(args, remaining, pm)
     if args.verbose > 2: