Shaohua Li <[email protected]> writes:
> plug is still helpful for workload with IO merge, but it can be harmful
> otherwise especially with multiple hardware queues, as there is
> (supposed) no lock contention in this case and plug can introduce
> latency. For multiple queues, we do limited plug, eg plug only if there
> is request merge. If a request doesn't have merge with following
> request, the requet will be dispatched immediately.
>
> This also fixes a bug. If we directly issue a request and it fails, we
> use blk_mq_merge_queue_io(). But we already assigned bio to a request in
> blk_mq_bio_to_request. blk_mq_merge_queue_io shouldn't run
> blk_mq_bio_to_request again.
Good catch. Might've been better to split that out first for easy
backport to stable kernels, but I won't hold you to that.
> @@ -1243,6 +1277,10 @@ static void blk_mq_make_request(struct request_queue
> *q, struct bio *bio)
> return;
> }
>
> + if (likely(!is_flush_fua) && !blk_queue_nomerges(q) &&
> + blk_attempt_plug_merge(q, bio, &request_count))
> + return;
> +
> rq = blk_mq_map_request(q, bio, &data);
> if (unlikely(!rq))
> return;
After this patch, everything up to this point in blk_mq_make_request and
blk_sq_make_request is the same. This can be factored out (in another
patch) to a common function.
> @@ -1253,38 +1291,38 @@ static void blk_mq_make_request(struct request_queue
> *q, struct bio *bio)
> goto run_queue;
> }
>
> + plug = current->plug;
> /*
> * If the driver supports defer issued based on 'last', then
> * queue it up like normal since we can potentially save some
> * CPU this way.
> */
> - if (is_sync && !(data.hctx->flags & BLK_MQ_F_DEFER_ISSUE)) {
> - struct blk_mq_queue_data bd = {
> - .rq = rq,
> - .list = NULL,
> - .last = 1
> - };
> - int ret;
> + if ((plug || is_sync) && !(data.hctx->flags & BLK_MQ_F_DEFER_ISSUE)) {
> + struct request *old_rq = NULL;
I would add a !blk_queue_nomerges(q) to that conditional. There's no
point holding back an I/O when we won't merge it anyway.
That brings up another quirk of the current implementation (not your
patches) that bugs me.
BLK_MQ_F_SHOULD_MERGE
QUEUE_FLAG_NOMERGES
Those two flags are set independently, one via the driver and the other
via a sysfs file. So the user could set the nomerges flag to 1 or 2,
and still potentially get merges (see blk_mq_merge_queue_io). That's
something that should be fixed, albeit that can wait.
> blk_mq_bio_to_request(rq, bio);
>
> /*
> - * For OK queue, we are done. For error, kill it. Any other
> - * error (busy), just add it to our list as we previously
> - * would have done
> + * we do limited pluging. If bio can be merged, do merge.
> + * Otherwise the existing request in the plug list will be
> + * issued. So the plug list will have one request at most
> */
> - ret = q->mq_ops->queue_rq(data.hctx, &bd);
> - if (ret == BLK_MQ_RQ_QUEUE_OK)
> - goto done;
> - else {
> - __blk_mq_requeue_request(rq);
> -
> - if (ret == BLK_MQ_RQ_QUEUE_ERROR) {
> - rq->errors = -EIO;
> - blk_mq_end_request(rq, rq->errors);
> - goto done;
> + if (plug) {
> + if (!list_empty(&plug->mq_list)) {
> + old_rq = list_first_entry(&plug->mq_list,
> + struct request, queuelist);
> + list_del_init(&old_rq->queuelist);
> }
> - }
> + list_add_tail(&rq->queuelist, &plug->mq_list);
> + } else /* is_sync */
> + old_rq = rq;
> + blk_mq_put_ctx(data.ctx);
> + if (!old_rq)
> + return;
> + if (!blk_mq_direct_issue_request(old_rq))
> + return;
> + blk_mq_insert_request(old_rq, false, true, true);
> + return;
> }
Now there is no way to exit that if block, we always return. It may be
worth cosidering moving that block to its own function, if you can think
of a good name for it.
Other than those minor issues, this looks good to me.
Cheers,
Jeff
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