throttle: Add throttle group support

The throttle group support use a cooperative round robin scheduling
algorithm.

The principles of the algorithm are simple:
- Each BDS of the group is used as a token in a circular way.
- The active BDS computes if a wait must be done and arms the right
  timer.
- If a wait must be done the token timer will be armed so the token
  will become the next active BDS.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: f0082a86f3ac01c46170f7eafe2101a92e8fde39.1433779731.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

include/block/block.h

@@ -173,8 +173,9 @@ void bdrv_stats_print(Monitor *mon, const QObject *data);
 void bdrv_info_stats(Monitor *mon, QObject **ret_data);
 
 /* disk I/O throttling */
-void bdrv_io_limits_enable(BlockDriverState *bs);
+void bdrv_io_limits_enable(BlockDriverState *bs, const char *group);
 void bdrv_io_limits_disable(BlockDriverState *bs);
+void bdrv_io_limits_update_group(BlockDriverState *bs, const char *group);
 
 void bdrv_init(void);
 void bdrv_init_with_whitelist(void);

include/block/block_int.h

@@ -379,10 +379,13 @@ struct BlockDriverState {
     unsigned int serialising_in_flight;
 
     /* I/O throttling */
-    ThrottleState throttle_state;
-    ThrottleTimers throttle_timers;
     CoQueue      throttled_reqs[2];
     bool         io_limits_enabled;
+    /* The following fields are protected by the ThrottleGroup lock.
+     * See the ThrottleGroup documentation for details. */
+    ThrottleState *throttle_state;
+    ThrottleTimers throttle_timers;
+    unsigned       pending_reqs[2];
     QLIST_ENTRY(BlockDriverState) round_robin;
 
     /* I/O stats (display with "info blockstats"). */

include/block/throttle-groups.h

@@ -36,4 +36,8 @@ void throttle_group_get_config(BlockDriverState *bs, ThrottleConfig *cfg);
 void throttle_group_register_bs(BlockDriverState *bs, const char *groupname);
 void throttle_group_unregister_bs(BlockDriverState *bs);
 
+void coroutine_fn throttle_group_co_io_limits_intercept(BlockDriverState *bs,
+                                                        unsigned int bytes,
+                                                        bool is_write);
+
 #endif