Signal timing problems in python testsuite

[Rolf Eike Beer <eike-kernel@xxxxxxxxx>]

The Python testsuite in all versions of Python3 I remember fails the eintr 
tests with output like this:

FAIL: test_poll (__main__.SelectEINTRTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.9/test/eintrdata/eintr_tester.py", line 451, in 
test_poll
    self.assertGreaterEqual(dt, self.sleep_time)
AssertionError: 0.19999999995343387 not greater than or equal to 0.2

There are 2 more failing tests with likewise output. This happens on my 4-core 
C8000. I have attached the testcase, when running it with "python3 
test_eintr.py" I also get that results.

I suspect this could be some cross-cpu clock scew, but of course it can be 
something entirely different. Could someone have a look what is up there? 
Kernel is currently 5.10.4, but this has been there for a while.

Eike

import os
import signal
import subprocess
import sys
import unittest

from test import support
from test.support import script_helper


@unittest.skipUnless(os.name == "posix", "only supported on Unix")
class EINTRTests(unittest.TestCase):

    @unittest.skipUnless(hasattr(signal, "setitimer"), "requires setitimer()")
    def test_all(self):
        # Run the tester in a sub-process, to make sure there is only one
        # thread (for reliable signal delivery).
        tester = support.findfile("eintr_tester.py", subdir="eintrdata")
        # use -u to try to get the full output if the test hangs or crash
        args = ["-u", tester, "-v"]
        if support.verbose:
            print()
            print("--- run eintr_tester.py ---", flush=True)
            # In verbose mode, the child process inherit stdout and stdout,
            # to see output in realtime and reduce the risk of losing output.
            args = [sys.executable, "-E", "-X", "faulthandler", *args]
            proc = subprocess.run(args)
            print(f"--- eintr_tester.py completed: "
                  f"exit code {proc.returncode} ---", flush=True)
            if proc.returncode:
                self.fail("eintr_tester.py failed")
        else:
            script_helper.assert_python_ok("-u", tester, "-v")


if __name__ == "__main__":
    unittest.main()

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