sbitmap: Advance the queue index before waking up a queue

When a queue is awaken, the wake_index written by sbq_wake_ptr currently
keeps pointing to the same queue.  On the next wake up, it will thus
retry the same queue, which is unfair to other queues, and can lead to
starvation.  This patch, moves the index update to happen before the
queue is returned, such that it will now try a different queue first on
the next wake up, improving fairness.

Fixes: 4f8126bb2308 ("sbitmap: Use single per-bitmap counting to wake up queued tags")
Reported-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Link: https://lore.kernel.org/r/20221115224553.23594-2-krisman@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index eca462cba3980d0f8fb9f2c608e7c070f2ff7402..bea7984f7987f3edc2362749fe9868b0535bc739 100644 (file)
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -571,13 +571,19 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
        for (i = 0; i < SBQ_WAIT_QUEUES; i++) {
                struct sbq_wait_state *ws = &sbq->ws[wake_index];
 
+               /*
+                * Advance the index before checking the current queue.
+                * It improves fairness, by ensuring the queue doesn't
+                * need to be fully emptied before trying to wake up
+                * from the next one.
+                */
+               wake_index = sbq_index_inc(wake_index);
+
                if (waitqueue_active(&ws->wait)) {
                        if (wake_index != atomic_read(&sbq->wake_index))
                                atomic_set(&sbq->wake_index, wake_index);
                        return ws;
                }
-
-               wake_index = sbq_index_inc(wake_index);
        }
 
        return NULL;