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blk-mq: don't allocate driver tag upfront for flush rq

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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 <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
This commit is contained in:
Ming Lei 2017-11-02 23:24:38 +08:00 • committed by Jens Axboe
commit 923218f616
3 changed files with 37 additions and 81 deletions
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35
block/blk-flush.c
View file

@ -231,8 +231,13 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
/* release the tag's ownership to the req cloned from */
spin_lock_irqsave(&fq->mq_flush_lock, flags);
hctx = blk_mq_map_queue(q, flush_rq->mq_ctx->cpu);
blk_mq_tag_set_rq(hctx, flush_rq->tag, fq->orig_rq);
flush_rq->tag = -1;
if (!q->elevator) {
blk_mq_tag_set_rq(hctx, flush_rq->tag, fq->orig_rq);
flush_rq->tag = -1;
} else {
blk_mq_put_driver_tag_hctx(hctx, flush_rq);
flush_rq->internal_tag = -1;
}
}
running = &fq->flush_queue[fq->flush_running_idx];
@ -318,19 +323,26 @@ static bool blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq)
blk_rq_init(q, flush_rq);
/*
* Borrow tag from the first request since they can't
* be in flight at the same time. And acquire the tag's
* ownership for flush req.
* In case of none scheduler, borrow tag from the first request
* since they can't be in flight at the same time. And acquire
* the tag's ownership for flush req.
*
* In case of IO scheduler, flush rq need to borrow scheduler tag
* just for cheating put/get driver tag.
*/
if (q->mq_ops) {
struct blk_mq_hw_ctx *hctx;
flush_rq->mq_ctx = first_rq->mq_ctx;
flush_rq->tag = first_rq->tag;
fq->orig_rq = first_rq;
hctx = blk_mq_map_queue(q, first_rq->mq_ctx->cpu);
blk_mq_tag_set_rq(hctx, first_rq->tag, flush_rq);
if (!q->elevator) {
fq->orig_rq = first_rq;
flush_rq->tag = first_rq->tag;
hctx = blk_mq_map_queue(q, first_rq->mq_ctx->cpu);
blk_mq_tag_set_rq(hctx, first_rq->tag, flush_rq);
} else {
flush_rq->internal_tag = first_rq->internal_tag;
}
}
flush_rq->cmd_flags = REQ_OP_FLUSH | REQ_PREFLUSH;
@ -394,6 +406,11 @@ static void mq_flush_data_end_io(struct request *rq, blk_status_t error)
hctx = blk_mq_map_queue(q, ctx->cpu);
if (q->elevator) {
WARN_ON(rq->tag < 0);
blk_mq_put_driver_tag_hctx(hctx, rq);
}
/*
* After populating an empty queue, kick it to avoid stall. Read
* the comment in flush_end_io().