Always yield to other greenthreads after database calls

Since common database backends (eg MySQLdb and sqlite) are implemented
in C extensions, eventlet cannot monkey patch the socket calls to ensure
greenthread context switches occur during network I/O.

Previously, this was done by sprinkling explicit greenthread.sleep(0)
calls in areas known to cause problems. Switch that to always do a
yield after execute.

Change-Id: If91e49fbe212a02710b6ba4abd456222552e2575

2 files changed, 14 insertions, 8 deletions

diff --git a/nova/compute/manager.py b/nova/compute/manager.py
index 2f977b26c..e4db4a3c2 100644
--- a/nova/compute/manager.py
+++ b/nova/compute/manager.py
@@ -2435,9 +2435,6 @@ class ComputeManager(manager.SchedulerDependentManager):
                                         {'status': 'error'})
 
             for migration in migrations:
-                # NOTE(comstud): Yield to other greenthreads.  Putting this
-                # at the top so we make sure to do it on each iteration.
-                greenthread.sleep(0)
                 migration_id = migration['id']
                 instance_uuid = migration['instance_uuid']
                 LOG.info(_("Automatically confirming migration "
@@ -2539,8 +2536,6 @@ class ComputeManager(manager.SchedulerDependentManager):
 
             refreshed = timeutils.utcnow()
             for usage in bw_usage:
-                # Allow switching of greenthreads between queries.
-                greenthread.sleep(0)
                 self.db.bw_usage_update(context,
                                         usage['uuid'],
                                         usage['mac_address'],
@@ -2584,8 +2579,6 @@ class ComputeManager(manager.SchedulerDependentManager):
                        "%(num_vm_instances)s on the hypervisor.") % locals())
 
         for db_instance in db_instances:
-            # Allow other periodic tasks to do some work...
-            greenthread.sleep(0)
             db_power_state = db_instance['power_state']
             if db_instance['task_state'] is not None:
                 LOG.info(_("During sync_power_state the instance has a "
diff --git a/nova/db/sqlalchemy/session.py b/nova/db/sqlalchemy/session.py
index 6e754be7e..6c349e0c2 100644
--- a/nova/db/sqlalchemy/session.py
+++ b/nova/db/sqlalchemy/session.py
@@ -21,6 +21,7 @@
 import re
 import time
 
+from eventlet import greenthread
 from sqlalchemy.exc import DisconnectionError, OperationalError
 import sqlalchemy.interfaces
 import sqlalchemy.orm
@@ -66,6 +67,16 @@ def add_regexp_listener(dbapi_con, con_record):
     dbapi_con.create_function('regexp', 2, regexp)
 
 
+def greenthread_yield(dbapi_con, con_record):
+    """
+    Ensure other greenthreads get a chance to execute by forcing a context
+    switch. With common database backends (eg MySQLdb and sqlite), there is
+    no implicit yield caused by network I/O since they are implemented by
+    C libraries that eventlet cannot monkey patch.
+    """
+    greenthread.sleep(0)
+
+
 def ping_listener(dbapi_conn, connection_rec, connection_proxy):
     """
     Ensures that MySQL connections checked out of the
@@ -122,9 +133,11 @@ def get_engine():
 
         _ENGINE = sqlalchemy.create_engine(FLAGS.sql_connection, **engine_args)
 
+        sqlalchemy.event.listen(_ENGINE, 'checkin', greenthread_yield)
+
         if 'mysql' in connection_dict.drivername:
             sqlalchemy.event.listen(_ENGINE, 'checkout', ping_listener)
-        elif "sqlite" in connection_dict.drivername:
+        elif 'sqlite' in connection_dict.drivername:
             if not FLAGS.sqlite_synchronous:
                 sqlalchemy.event.listen(_ENGINE, 'connect',
                                         synchronous_switch_listener)