contest: vm: use select() in drain_to_prompt()

We see possible issues with the txtimestamp test related to IO
blocking the test for a long time. Our current scheme is to poll
the io every 30msec, which is not very often. Improve the code
by using select() to react to pending input more quickly.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

contest/remote/lib/vm.py

@@ -3,14 +3,15 @@
 import unicodedata
 import requests
 import subprocess
-from time import sleep
 import fcntl
 import json
 import os
 import psutil
 import re
+import select
 import shutil
 import signal
+import time
 from .crash import has_crash, extract_crash
 
 
@@ -301,7 +302,10 @@ def drain_to_prompt(self, prompt="xx__-> ", dump_after=None, deadline=None):
         total_wait = 0
         stdout = ""
         stderr = ""
+        last_read = time.monotonic()
         while True:
+            select.select([self.p.stdout, self.p.stderr], [], [], 0.1)
+
             read_some, out = self._read_pipe_nonblock(self.p.stdout)
             self.log_out += out
             stdout += out
@@ -310,6 +314,11 @@ def drain_to_prompt(self, prompt="xx__-> ", dump_after=None, deadline=None):
             self.log_err += err
             stderr += err
 
+            now = time.monotonic()
+            elapsed = now - last_read
+            last_read = now
+            total_wait += elapsed
+
             if read_some:
                 if stdout.endswith(prompt):
                     break
@@ -322,12 +331,10 @@ def drain_to_prompt(self, prompt="xx__-> ", dump_after=None, deadline=None):
                 # logs elsewhere try to get a new prompt by sending a new line.
                 if prompt in out:
                     self.cmd('\n')
-                    sleep(0.25)
+                    time.sleep(0.25)
                 waited = 0
             else:
-                total_wait += 0.03
-                waited += 0.03
-                sleep(0.03)
+                waited += elapsed
 
             if total_wait > hard_stop:
                 self.log_err += f'\nHARD STOP ({hard_stop})\n'
@@ -413,7 +420,7 @@ def check_health(self):
             self.drain_to_prompt()
             # kmemleak needs objects to be at least MSECS_MIN_AGE (5000)
             # before it considers them to have been leaked
-            sleep(5)
+            time.sleep(5)
             # Second scan, to identify what has really leaked
             self.cmd("echo scan > /sys/kernel/debug/kmemleak && cat /sys/kernel/debug/kmemleak")
             self.drain_to_prompt()