[RFC PATCH] sbitmap: fix batching wakeup

[Ming Lei <ming.lei@redhat.com>]

From: David Jeffery <djeffery@redhat.com>

Current code supposes that it is enough to provide forward progress by just
waking up one wait queue after one completion batch is done.

Unfortunately this way isn't enough, cause waiter can be added to
wait queue just after it is woken up.

Follows one example(64 depth, wake_batch is 8)

1) all 64 tags are active

2) in each wait queue, there is only one single waiter

3) each time one completion batch(8 completions) wakes up just one waiter in each wait
queue, then immediately one new sleeper is added to this wait queue

4) after 64 completions, 8 waiters are wakeup, and there are still 8 waiters in each
wait queue

5) after another 8 active tags are completed, only one waiter can be wakeup, and the other 7
can't be waken up anymore.

Turns out it isn't easy to fix this problem, so simply wakeup enough waiters for
single batch.

Cc: David Jeffery <djeffery@redhat.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Gabriel Krisman Bertazi <krisman@suse.de>
Cc: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 lib/sbitmap.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index eff4e42c425a..d0a5081dfd12 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -550,7 +550,7 @@ EXPORT_SYMBOL_GPL(sbitmap_queue_min_shallow_depth);
 
 static void __sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
 {
-	int i, wake_index;
+	int i, wake_index, woken;
 
 	if (!atomic_read(&sbq->ws_active))
 		return;
@@ -567,13 +567,12 @@ static void __sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
 		 */
 		wake_index = sbq_index_inc(wake_index);
 
-		/*
-		 * It is sufficient to wake up at least one waiter to
-		 * guarantee forward progress.
-		 */
-		if (waitqueue_active(&ws->wait) &&
-		    wake_up_nr(&ws->wait, nr))
-			break;
+		if (waitqueue_active(&ws->wait)) {
+			woken = wake_up_nr(&ws->wait, nr);
+			if (woken == nr)
+				break;
+			nr -= woken;
+		}
 	}
 
 	if (wake_index != atomic_read(&sbq->wake_index))
-- 
2.40.1