[PATCH net-next 2/2] selftests: net: py: don't default to shell=True

[Jakub Kicinski <kuba@kernel.org>]

Overhead of using shell=True is quite significant.
Micro-benchmark of running ethtool --help shows that
non-shell run is 2x faster.

Runtime of the XDP tests also shows improvement:
this patch: 2m34s 2m21s 2m18s 2m18s
    before:     2m54s 2m36s 2m34s

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 tools/testing/selftests/net/lib/py/utils.py | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/net/lib/py/utils.py b/tools/testing/selftests/net/lib/py/utils.py
index b188cac49738..e7155b6db9a3 100644
--- a/tools/testing/selftests/net/lib/py/utils.py
+++ b/tools/testing/selftests/net/lib/py/utils.py
@@ -29,9 +29,12 @@ import time
     """
     Execute a command on local or remote host.
 
+    @shell defaults to false, and class will try to split @comm into a list
+    if it's a string with spaces.
+
     Use bkg() instead to run a command in the background.
     """
-    def __init__(self, comm, shell=True, fail=True, ns=None, background=False,
+    def __init__(self, comm, shell=None, fail=True, ns=None, background=False,
                  host=None, timeout=5, ksft_wait=None):
         if ns:
             comm = f'ip netns exec {ns} ' + comm
@@ -45,6 +48,10 @@ import time
         if host:
             self.proc = host.cmd(comm)
         else:
+            # If user doesn't explicitly request shell try to avoid it.
+            if shell is None and isinstance(comm, str) and ' ' in comm:
+                comm = comm.split()
+
             # ksft_wait lets us wait for the background process to fully start,
             # we pass an FD to the child process, and wait for it to write back.
             # Similarly term_fd tells child it's time to exit.
@@ -111,7 +118,7 @@ import time
 
         with bkg("my_binary", ksft_wait=5):
     """
-    def __init__(self, comm, shell=True, fail=None, ns=None, host=None,
+    def __init__(self, comm, shell=None, fail=None, ns=None, host=None,
                  exit_wait=False, ksft_wait=None):
         super().__init__(comm, background=True,
                          shell=shell, fail=fail, ns=ns, host=host,
-- 
2.51.0