blk-mq: Explain when 'active_queues' is decremented

It is nontrivial to derive from the blk-mq source code when
blk_mq_tags.active_queues is decremented. Hence add a comment that
explains this.

Signed-off-by: Bart Van Assche <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Hannes Reinecke <[email protected]>
Cc: Johannes Thumshirn <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 9aa24c9508f908389f4e78ca04e755fd3419f01c..266fc4f6b046f29cc6c1f45d476232880fc0ecfe 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -954,6 +954,12 @@ static void blk_mq_timeout_work(struct work_struct *work)
                data.next = blk_rq_timeout(round_jiffies_up(data.next));
                mod_timer(&q->timeout, data.next);
        } else {
+               /*
+                * Request timeouts are handled as a forward rolling timer. If
+                * we end up here it means that no requests are pending and
+                * also that no request has been pending for a while. Mark
+                * each hctx as idle.
+                */
                queue_for_each_hw_ctx(q, hctx, i) {
                        /* the hctx may be unmapped, so check it here */
                        if (blk_mq_hw_queue_mapped(hctx))