From 923218f6166a84688973acdc39094f3bee1e9ad4 Mon Sep 17 00:00:00 2001 From: Ming Lei Date: Thu, 2 Nov 2017 23:24:38 +0800 Subject: blk-mq: don't allocate driver tag upfront for flush rq The idea behind it is simple: 1) for none scheduler, driver tag has to be borrowed for flush rq, otherwise we may run out of tag, and that causes an IO hang. And get/put driver tag is actually noop for none, so reordering tags isn't necessary at all. 2) for a real I/O scheduler, we need not allocate a driver tag upfront for flush rq. It works just fine to follow the same approach as normal requests: allocate driver tag for each rq just before calling ->queue_rq(). One driver visible change is that the driver tag isn't shared in the flush request sequence. That won't be a problem, since we always do that in legacy path. Then flush rq need not be treated specially wrt. get/put driver tag. This cleans up the code - for instance, reorder_tags_to_front() can be removed, and we needn't worry about request ordering in dispatch list for avoiding I/O deadlock. Also we have to put the driver tag before requeueing. Signed-off-by: Ming Lei Signed-off-by: Jens Axboe --- block/blk-mq.c | 41 ++++++----------------------------------- 1 file changed, 6 insertions(+), 35 deletions(-) (limited to 'block/blk-mq.c') diff --git a/block/blk-mq.c b/block/blk-mq.c index 14f6886fbec8..c501cbd0de93 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -653,6 +653,8 @@ static void __blk_mq_requeue_request(struct request *rq) { struct request_queue *q = rq->q; + blk_mq_put_driver_tag(rq); + trace_block_rq_requeue(q, rq); wbt_requeue(q->rq_wb, &rq->issue_stat); blk_mq_sched_requeue_request(rq); @@ -996,30 +998,6 @@ done: return rq->tag != -1; } -/* - * If we fail getting a driver tag because all the driver tags are already - * assigned and on the dispatch list, BUT the first entry does not have a - * tag, then we could deadlock. For that case, move entries with assigned - * driver tags to the front, leaving the set of tagged requests in the - * same order, and the untagged set in the same order. - */ -static bool reorder_tags_to_front(struct list_head *list) -{ - struct request *rq, *tmp, *first = NULL; - - list_for_each_entry_safe_reverse(rq, tmp, list, queuelist) { - if (rq == first) - break; - if (rq->tag != -1) { - list_move(&rq->queuelist, list); - if (!first) - first = rq; - } - } - - return first != NULL; -} - static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode, int flags, void *key) { @@ -1080,9 +1058,6 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, rq = list_first_entry(list, struct request, queuelist); if (!blk_mq_get_driver_tag(rq, &hctx, false)) { - if (!queued && reorder_tags_to_front(list)) - continue; - /* * The initial allocation attempt failed, so we need to * rerun the hardware queue when a tag is freed. @@ -1133,7 +1108,6 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, nxt = list_first_entry(list, struct request, queuelist); blk_mq_put_driver_tag(nxt); } - blk_mq_put_driver_tag_hctx(hctx, rq); list_add(&rq->queuelist, list); __blk_mq_requeue_request(rq); break; @@ -1698,13 +1672,10 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio) if (unlikely(is_flush_fua)) { blk_mq_put_ctx(data.ctx); blk_mq_bio_to_request(rq, bio); - if (q->elevator) { - blk_mq_sched_insert_request(rq, false, true, true, - true); - } else { - blk_insert_flush(rq); - blk_mq_run_hw_queue(data.hctx, true); - } + + /* bypass scheduler for flush rq */ + blk_insert_flush(rq); + blk_mq_run_hw_queue(data.hctx, true); } else if (plug && q->nr_hw_queues == 1) { struct request *last = NULL; -- cgit v1.2.3