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Date: Tue, 10 Jan 2023 20:35:02 +0800
From: Kemeng Shi <shikemeng@...weicloud.com>
To: Christoph Hellwig <hch@....de>
Cc: axboe@...nel.dk, dwagner@...e.de, hare@...e.de,
ming.lei@...hat.com, linux-block@...r.kernel.org,
linux-kernel@...r.kernel.org, john.garry@...wei.com, jack@...e.cz
Subject: Re: [PATCH v2 08/13] blk-mq: simplify flush check in
blk_mq_dispatch_rq_list
on 1/10/2023 4:09 PM, Christoph Hellwig wrote:
> On Mon, Jan 09, 2023 at 10:27:33AM +0800, Kemeng Shi wrote:
>> After we queue request[s] to one driver queue, we need to notify driver
>> that there are no more request to the queue or driver will keep waiting
>> for the last request to be queued and IO hung could happen.
>
> Yes.
>
>> Normaly, we will notify this by setting .last in struct blk_mq_queue_data
>> along with the normal last request .rq in struct blk_mq_queue_data. The
>> extra commit is only needed if normal last information in .last is lost.
>> (See comment in struct blk_mq_ops for commit_rqs).
>>
>> The lost could occur if error happens for sending last request with .last
>> set or error happen in middle of list and we even do not send the request
>> with .last set.
>
> Yes. So the rule is:
>
> 1) did not queue everything initially scheduled to queue
>
> OR
>
> 2) the last attempt to queue a request failed
>
> I think we need to find a way to clearly document that and that
> make all callers match it.
> For most this becomes a
>
> if (ret || !list_empty(list))
>
> or even just
>
> if (ret)
>
> as an error is often the only way to break out of the submission
> loop.
>
> I wonder if we need to split the queued clearing from blk_mq_commit_rqs
> and just clear it in the existing callers, so that we can use that
> helpers for all commits, nicely hiding the ->commit_rqs presence
> check, and then move that call to where it is needed directly. Something
> like this untested patch (which needs to be split up), which also
> makes sure we trace these calls consistently:
Yes, using helper also makes queued check consistently. Currently, most code
only calls commit_rqs if any request is queued, one exception is that
blk_mq_plug_issue_direct calls commit_rqs without queued check.
Besides, we can document the the rule before blk_mq_commit_rqs. Any caller
in future can notice the rule and match it.
I will send next version based on suggested helper.
Thanks.
> ---
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index c5cf0dbca1db8d..436ca56a0b7172 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2001,6 +2001,15 @@ static void blk_mq_release_budgets(struct request_queue *q,
> }
> }
>
> +static void blk_mq_commit_rqs(struct blk_mq_hw_ctx *hctx, int queued,
> + bool from_schedule)
> +{
> + if (queued && hctx->queue->mq_ops->commit_rqs) {
> + trace_block_unplug(hctx->queue, queued, !from_schedule);
> + hctx->queue->mq_ops->commit_rqs(hctx);
> + }
> +}
> +
> /*
> * Returns true if we did some work AND can potentially do more.
> */
> @@ -2082,12 +2091,9 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list,
> if (!list_empty(&zone_list))
> list_splice_tail_init(&zone_list, list);
>
> - /* If we didn't flush the entire list, we could have told the driver
> - * there was more coming, but that turned out to be a lie.
> - */
> - if ((!list_empty(list) || errors || needs_resource ||
> - ret == BLK_STS_DEV_RESOURCE) && q->mq_ops->commit_rqs && queued)
> - q->mq_ops->commit_rqs(hctx);
> + if (!list_empty(list) || ret)
> + blk_mq_commit_rqs(hctx, queued, false);
> +
> /*
> * Any items that need requeuing? Stuff them into hctx->dispatch,
> * that is where we will continue on next queue run.
> @@ -2548,16 +2554,6 @@ void blk_mq_insert_requests(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
> spin_unlock(&ctx->lock);
> }
>
> -static void blk_mq_commit_rqs(struct blk_mq_hw_ctx *hctx, int *queued,
> - bool from_schedule)
> -{
> - if (hctx->queue->mq_ops->commit_rqs) {
> - trace_block_unplug(hctx->queue, *queued, !from_schedule);
> - hctx->queue->mq_ops->commit_rqs(hctx);
> - }
> - *queued = 0;
> -}
> -
> static void blk_mq_bio_to_request(struct request *rq, struct bio *bio,
> unsigned int nr_segs)
> {
> @@ -2684,17 +2680,17 @@ static blk_status_t blk_mq_request_issue_directly(struct request *rq, bool last)
> static void blk_mq_plug_issue_direct(struct blk_plug *plug, bool from_schedule)
> {
> struct blk_mq_hw_ctx *hctx = NULL;
> + blk_status_t ret = BLK_STS_OK;
> struct request *rq;
> int queued = 0;
> - int errors = 0;
>
> while ((rq = rq_list_pop(&plug->mq_list))) {
> bool last = rq_list_empty(plug->mq_list);
> - blk_status_t ret;
>
> if (hctx != rq->mq_hctx) {
> if (hctx)
> - blk_mq_commit_rqs(hctx, &queued, from_schedule);
> + blk_mq_commit_rqs(hctx, queued, from_schedule);
> + queued = 0;
> hctx = rq->mq_hctx;
> }
>
> @@ -2706,21 +2702,15 @@ static void blk_mq_plug_issue_direct(struct blk_plug *plug, bool from_schedule)
> case BLK_STS_RESOURCE:
> case BLK_STS_DEV_RESOURCE:
> blk_mq_request_bypass_insert(rq, false, true);
> - blk_mq_commit_rqs(hctx, &queued, from_schedule);
> - return;
> + goto out;
> default:
> blk_mq_end_request(rq, ret);
> - errors++;
> break;
> }
> }
> -
> - /*
> - * If we didn't flush the entire list, we could have told the driver
> - * there was more coming, but that turned out to be a lie.
> - */
> - if (errors)
> - blk_mq_commit_rqs(hctx, &queued, from_schedule);
> +out:
> + if (ret)
> + blk_mq_commit_rqs(hctx, queued, from_schedule);
> }
>
> static void __blk_mq_flush_plug_list(struct request_queue *q,
> @@ -2804,37 +2794,33 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
> void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
> struct list_head *list)
> {
> + blk_status_t ret = BLK_STS_OK;
> + struct request *rq;
> int queued = 0;
> - int errors = 0;
> -
> - while (!list_empty(list)) {
> - blk_status_t ret;
> - struct request *rq = list_first_entry(list, struct request,
> - queuelist);
> + bool last;
>
> + while ((rq = list_first_entry_or_null(list, struct request,
> + queuelist))) {
> list_del_init(&rq->queuelist);
> - ret = blk_mq_request_issue_directly(rq, list_empty(list));
> - if (ret != BLK_STS_OK) {
> - errors++;
> - if (ret == BLK_STS_RESOURCE ||
> - ret == BLK_STS_DEV_RESOURCE) {
> - blk_mq_request_bypass_insert(rq, false,
> - list_empty(list));
> - break;
> - }
> - blk_mq_end_request(rq, ret);
> - } else
> + last = list_empty(list);
> +
> + ret = blk_mq_request_issue_directly(rq, last);
> + switch (ret) {
> + case BLK_STS_OK:
> queued++;
> + break;
> + case BLK_STS_RESOURCE:
> + case BLK_STS_DEV_RESOURCE:
> + blk_mq_request_bypass_insert(rq, false, last);
> + goto out;
> + default:
> + blk_mq_end_request(rq, ret);
> + break;
> + }
> }
> -
> - /*
> - * If we didn't flush the entire list, we could have told
> - * the driver there was more coming, but that turned out to
> - * be a lie.
> - */
> - if ((!list_empty(list) || errors) &&
> - hctx->queue->mq_ops->commit_rqs && queued)
> - hctx->queue->mq_ops->commit_rqs(hctx);
> +out:
> + if (ret)
> + blk_mq_commit_rqs(hctx, queued, false);
> }
>
> static bool blk_mq_attempt_bio_merge(struct request_queue *q,
>
--
Best wishes
Kemeng Shi
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