block: introduce bdrv_no_throttling_begin/end

Extract the handling of throttling from bdrv_flush_io_queue.  These
new functions will soon become BdrvChildRole callbacks, as they can
be generalized to "beginning of drain" and "end of drain".

Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -2261,7 +2261,6 @@ static void swap_feature_fields(BlockDriverState *bs_top,
 
     assert(!bs_new->throttle_state);
     if (bs_top->throttle_state) {
-        assert(bs_top->io_limits_enabled);
         bdrv_io_limits_enable(bs_new, throttle_group_get_name(bs_top));
         bdrv_io_limits_disable(bs_top);
     }

block/block-backend.c

@@ -794,7 +794,6 @@ int blk_read_unthrottled(BlockBackend *blk, int64_t sector_num, uint8_t *buf,
                          int nb_sectors)
 {
     BlockDriverState *bs = blk_bs(blk);
-    bool enabled;
     int ret;
 
     ret = blk_check_request(blk, sector_num, nb_sectors);
@@ -802,10 +801,9 @@ int blk_read_unthrottled(BlockBackend *blk, int64_t sector_num, uint8_t *buf,
         return ret;
     }
 
-    enabled = bs->io_limits_enabled;
-    bs->io_limits_enabled = false;
+    bdrv_no_throttling_begin(bs);
     ret = blk_read(blk, sector_num, buf, nb_sectors);
-    bs->io_limits_enabled = enabled;
+    bdrv_no_throttling_end(bs);
     return ret;
 }
 

block/io.c

@@ -65,28 +65,32 @@ void bdrv_set_io_limits(BlockDriverState *bs,
     throttle_group_config(bs, cfg);
 }
 
-static void bdrv_start_throttled_reqs(BlockDriverState *bs)
+void bdrv_no_throttling_begin(BlockDriverState *bs)
 {
-    bool enabled = bs->io_limits_enabled;
-
-    bs->io_limits_enabled = false;
+    if (bs->io_limits_disabled++ == 0) {
         throttle_group_restart_bs(bs);
-    bs->io_limits_enabled = enabled;
+    }
+}
+
+void bdrv_no_throttling_end(BlockDriverState *bs)
+{
+    assert(bs->io_limits_disabled);
+    --bs->io_limits_disabled;
 }
 
 void bdrv_io_limits_disable(BlockDriverState *bs)
 {
-    bs->io_limits_enabled = false;
-    bdrv_start_throttled_reqs(bs);
+    assert(bs->throttle_state);
+    bdrv_no_throttling_begin(bs);
     throttle_group_unregister_bs(bs);
+    bdrv_no_throttling_end(bs);
 }
 
 /* should be called before bdrv_set_io_limits if a limit is set */
 void bdrv_io_limits_enable(BlockDriverState *bs, const char *group)
 {
-    assert(!bs->io_limits_enabled);
+    assert(!bs->throttle_state);
     throttle_group_register_bs(bs, group);
-    bs->io_limits_enabled = true;
 }
 
 void bdrv_io_limits_update_group(BlockDriverState *bs, const char *group)
@@ -302,18 +306,22 @@ static void coroutine_fn bdrv_co_yield_to_drain(BlockDriverState *bs)
  */
 void coroutine_fn bdrv_co_drain(BlockDriverState *bs)
 {
+    bdrv_no_throttling_begin(bs);
     bdrv_drain_recurse(bs);
     bdrv_co_yield_to_drain(bs);
+    bdrv_no_throttling_end(bs);
 }
 
 void bdrv_drain(BlockDriverState *bs)
 {
+    bdrv_no_throttling_begin(bs);
     bdrv_drain_recurse(bs);
     if (qemu_in_coroutine()) {
         bdrv_co_yield_to_drain(bs);
     } else {
         bdrv_drain_poll(bs);
     }
+    bdrv_no_throttling_end(bs);
 }
 
 /*
@@ -336,6 +344,7 @@ void bdrv_drain_all(void)
         if (bs->job) {
             block_job_pause(bs->job);
         }
+        bdrv_no_throttling_begin(bs);
         bdrv_drain_recurse(bs);
         aio_context_release(aio_context);
 
@@ -377,6 +386,7 @@ void bdrv_drain_all(void)
         AioContext *aio_context = bdrv_get_aio_context(bs);
 
         aio_context_acquire(aio_context);
+        bdrv_no_throttling_end(bs);
         if (bs->job) {
             block_job_resume(bs->job);
         }
@@ -980,7 +990,7 @@ int coroutine_fn bdrv_co_do_preadv(BlockDriverState *bs,
     }
 
     /* throttling disk I/O */
-    if (bs->io_limits_enabled) {
+    if (bs->throttle_state) {
         throttle_group_co_io_limits_intercept(bs, bytes, false);
     }
 
@@ -1330,7 +1340,7 @@ int coroutine_fn bdrv_co_do_pwritev(BlockDriverState *bs,
     }
 
     /* throttling disk I/O */
-    if (bs->io_limits_enabled) {
+    if (bs->throttle_state) {
         throttle_group_co_io_limits_intercept(bs, bytes, true);
     }
 
@@ -2772,7 +2782,6 @@ void bdrv_flush_io_queue(BlockDriverState *bs)
     } else if (bs->file) {
         bdrv_flush_io_queue(bs->file->bs);
     }
-    bdrv_start_throttled_reqs(bs);
 }
 
 void bdrv_drained_begin(BlockDriverState *bs)

block/throttle-groups.c

@@ -219,6 +219,10 @@ static bool throttle_group_schedule_timer(BlockDriverState *bs,
     ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
     bool must_wait;
 
+    if (bs->io_limits_disabled) {
+        return false;
+    }
+
     /* Check if any of the timers in this group is already armed */
     if (tg->any_timer_armed[is_write]) {
         return true;

include/block/block_int.h

@@ -424,10 +424,10 @@ struct BlockDriverState {
 
     /* I/O throttling.
      * throttle_state tells us if this BDS has I/O limits configured.
-     * io_limits_enabled tells us if they are currently being
-     * enforced, but it can be temporarily set to false */
+     * io_limits_disabled tells us if they are currently being enforced */
     CoQueue      throttled_reqs[2];
-    bool         io_limits_enabled;
+    unsigned int io_limits_disabled;
+
     /* The following fields are protected by the ThrottleGroup lock.
      * See the ThrottleGroup documentation for details. */
     ThrottleState *throttle_state;
@@ -713,6 +713,9 @@ BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
                                   const BdrvChildRole *child_role);
 void bdrv_root_unref_child(BdrvChild *child);
 
+void bdrv_no_throttling_begin(BlockDriverState *bs);
+void bdrv_no_throttling_end(BlockDriverState *bs);
+
 void blk_dev_change_media_cb(BlockBackend *blk, bool load);
 bool blk_dev_has_removable_media(BlockBackend *blk);
 bool blk_dev_has_tray(BlockBackend *blk);