cpu affinity: try to continue even if cpus seem busy

- when the set of free cpus is empty, previous code died already when
  attempting to set the affinity in filter_available_cpus()
- warn but do not abort if auto-detection indicates that available cpus
  are already occupied. it could be zombies or debug instances..

 when the set of free cpus was empty

kafl_fuzzer/common/util.py

@@ -40,18 +40,18 @@ def get_qemu_processes():
     if (len(pids) > 0):
         logger.warn(msg + " " + repr(pids))
 
-# limit ourselves to CPUs not hogged by possible other qemu instances
+# filter available CPUs by those with existing qemu instances
 def filter_available_cpus():
     def get_qemu_processes():
         for proc in psutil.process_iter(['pid', 'name']):
             if 'qemu-system-x86_64' in proc.info['name']:
                 yield (proc.info['pid'])
 
-    cpus = os.sched_getaffinity(0)
+    avail = os.sched_getaffinity(0)
+    used = set()
     for pid in get_qemu_processes():
-        cpus -= os.sched_getaffinity(pid)
-    os.sched_setaffinity(0,cpus)
-    return cpus
+        used |= os.sched_getaffinity(pid)
+    return avail, used
 
 # pretty-printed hexdump
 def hexdump(src, length=16):

kafl_fuzzer/manager/core.py

@@ -15,6 +15,7 @@
 
 import multiprocessing
 import time
+import os
 import sys
 import logging
 from pprint import pformat
@@ -75,10 +76,17 @@ def start(config):
     if not config.ip0:
         logger.warn("No PT trace region defined.")
 
-    cpus = filter_available_cpus()
-    if num_worker > len(cpus):
-        logger.error(f"Requested {num_worker} but only {len(cpus)} free CPUs detected. Abort.")
-        return -1
+    avail, used = filter_available_cpus()
+    if num_worker > len(avail):
+        logger.error(f"Requested {num_worker} workers but only {len(avail)} vCPUs detected.")
+        return 1
+
+    # warn if we cannot limit ourselves to CPUs detected as free..
+    if num_worker + 1 >= len(avail-used):
+        logger.warn(f"Warning: Requested {num_worker} workers but {len(used)} out of {len(avail)} vCPUs seem busy?")
+        time.sleep(2)
+    else:
+        os.sched_setaffinity(0, avail-used)
 
     manager = ManagerTask(config)