xfs: avoid busy loops in GCD

When GCD has no new work to handle, but read, write or reset commands
are outstanding, it currently busy loops, which is a bit suboptimal,
and can lead to softlockup warnings in case of stuck commands.

Change the code so that the task state is only set to running when work
is performed, which looks a bit tricky due to the design of the
reading/writing/resetting lists that contain both in-flight and finished
commands.

Fixes: 080d01c ("xfs: implement zoned garbage collection")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

Author: Christoph Hellwig

fs/xfs/xfs_zone_gc.c

@@ -491,21 +491,6 @@ xfs_zone_gc_select_victim(
 	struct xfs_rtgroup	*victim_rtg = NULL;
 	unsigned int		bucket;
 
-	if (xfs_is_shutdown(mp))
-		return false;
-
-	if (iter->victim_rtg)
-		return true;
-
-	/*
-	 * Don't start new work if we are asked to stop or park.
-	 */
-	if (kthread_should_stop() || kthread_should_park())
-		return false;
-
-	if (!xfs_zoned_need_gc(mp))
-		return false;
-
 	spin_lock(&zi->zi_used_buckets_lock);
 	for (bucket = 0; bucket < XFS_ZONE_USED_BUCKETS; bucket++) {
 		victim_rtg = xfs_zone_gc_pick_victim_from(mp, bucket);
@@ -975,14 +960,35 @@ xfs_zone_gc_reset_zones(
 	} while (next);
 }
 
+static bool
+xfs_zone_gc_should_start_new_work(
+	struct xfs_zone_gc_data	*data)
+{
+	if (xfs_is_shutdown(data->mp))
+		return false;
+	if (!xfs_zone_gc_space_available(data))
+		return false;
+
+	if (!data->iter.victim_rtg) {
+		if (kthread_should_stop() || kthread_should_park())
+			return false;
+		if (!xfs_zoned_need_gc(data->mp))
+			return false;
+		if (!xfs_zone_gc_select_victim(data))
+			return false;
+	}
+
+	return true;
+}
+
 /*
  * Handle the work to read and write data for GC and to reset the zones,
  * including handling all completions.
  *
  * Note that the order of the chunks is preserved so that we don't undo the
  * optimal order established by xfs_zone_gc_query().
  */
-static bool
+static void
 xfs_zone_gc_handle_work(
 	struct xfs_zone_gc_data	*data)
 {
@@ -996,46 +1002,41 @@ xfs_zone_gc_handle_work(
 	zi->zi_reset_list = NULL;
 	spin_unlock(&zi->zi_reset_list_lock);
 
-	if (!xfs_zone_gc_select_victim(data) ||
-	    !xfs_zone_gc_space_available(data)) {
-		if (list_empty(&data->reading) &&
-		    list_empty(&data->writing) &&
-		    list_empty(&data->resetting) &&
-		    !reset_list)
-			return false;
-	}
-
-	__set_current_state(TASK_RUNNING);
-	try_to_freeze();
-
-	if (reset_list)
+	if (reset_list) {
+		set_current_state(TASK_RUNNING);
 		xfs_zone_gc_reset_zones(data, reset_list);
+	}
 
 	list_for_each_entry_safe(chunk, next, &data->resetting, entry) {
 		if (READ_ONCE(chunk->state) != XFS_GC_BIO_DONE)
 			break;
+		set_current_state(TASK_RUNNING);
 		xfs_zone_gc_finish_reset(chunk);
 	}
 
 	list_for_each_entry_safe(chunk, next, &data->writing, entry) {
 		if (READ_ONCE(chunk->state) != XFS_GC_BIO_DONE)
 			break;
+		set_current_state(TASK_RUNNING);
 		xfs_zone_gc_finish_chunk(chunk);
 	}
 
 	blk_start_plug(&plug);
 	list_for_each_entry_safe(chunk, next, &data->reading, entry) {
 		if (READ_ONCE(chunk->state) != XFS_GC_BIO_DONE)
 			break;
+		set_current_state(TASK_RUNNING);
 		xfs_zone_gc_write_chunk(chunk);
 	}
 	blk_finish_plug(&plug);
 
-	blk_start_plug(&plug);
-	while (xfs_zone_gc_start_chunk(data))
-		;
-	blk_finish_plug(&plug);
-	return true;
+	if (xfs_zone_gc_should_start_new_work(data)) {
+		set_current_state(TASK_RUNNING);
+		blk_start_plug(&plug);
+		while (xfs_zone_gc_start_chunk(data))
+			;
+		blk_finish_plug(&plug);
+	}
 }
 
 /*
@@ -1059,8 +1060,18 @@ xfs_zoned_gcd(
 	for (;;) {
 		set_current_state(TASK_INTERRUPTIBLE | TASK_FREEZABLE);
 		xfs_set_zonegc_running(mp);
-		if (xfs_zone_gc_handle_work(data))
+
+		xfs_zone_gc_handle_work(data);
+
+		/*
+		 * Only sleep if nothing set the state to running.  Else check for
+		 * work again as someone might have queued up more work and woken
+		 * us in the meantime.
+		 */
+		if (get_current_state() == TASK_RUNNING) {
+			try_to_freeze();
 			continue;
+		}
 
 		if (list_empty(&data->reading) &&
 		    list_empty(&data->writing) &&