block: optimise blk_flush_plug_list

Don't call flush_plug_callbacks if there are no plug callbacks.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
[hch: split from a larger patch]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211020144119.142582-4-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-core.c b/block/blk-core.c
index 88752e5..db8b2fe 100644 (file)
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1639,8 +1639,8 @@ EXPORT_SYMBOL(blk_check_plugged);
 
 void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 {
-       flush_plug_callbacks(plug, from_schedule);
-
+       if (!list_empty(&plug->cb_list))
+               flush_plug_callbacks(plug, from_schedule);
        if (!rq_list_empty(plug->mq_list))
                blk_mq_flush_plug_list(plug, from_schedule);
        if (unlikely(!from_schedule && plug->cached_rq))