Re: [PATCH 3/3] blk-mq: update hctx->nr_active in blk_mq_end_request_batch()

[Jens Axboe <axboe@kernel.dk>]

On 11/2/21 7:35 AM, Ming Lei wrote:
> In case of shared tags and none io sched, batched completion still may
> be run into, and hctx->nr_active is accounted when getting driver tag,
> so it has to be updated in blk_mq_end_request_batch().
> 
> Otherwise, hctx->nr_active may become same with queue depth, then
> hctx_may_queue() always return false, then io hang is caused.
> 
> Fixes the issue by updating the counter in batched way.
> 
> Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
> Fixes: f794f3351f26 ("block: add support for blk_mq_end_request_batch()")
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/blk-mq.c | 15 +++++++++++++--
>  block/blk-mq.h | 12 +++++++++---
>  2 files changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 07eb1412760b..0dbe75034f61 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -825,6 +825,7 @@ void blk_mq_end_request_batch(struct io_comp_batch *iob)
>  	struct blk_mq_hw_ctx *cur_hctx = NULL;
>  	struct request *rq;
>  	u64 now = 0;
> +	int active = 0;
>  
>  	if (iob->need_ts)
>  		now = ktime_get_ns();
> @@ -846,16 +847,26 @@ void blk_mq_end_request_batch(struct io_comp_batch *iob)
>  		rq_qos_done(rq->q, rq);
>  
>  		if (nr_tags == TAG_COMP_BATCH || cur_hctx != rq->mq_hctx) {
> -			if (cur_hctx)
> +			if (cur_hctx) {
> +				if (active)
> +					__blk_mq_sub_active_requests(cur_hctx,
> +							active);
>  				blk_mq_flush_tag_batch(cur_hctx, tags, nr_tags);
> +			}
>  			nr_tags = 0;
> +			active = 0;
>  			cur_hctx = rq->mq_hctx;
>  		}
>  		tags[nr_tags++] = rq->tag;
> +		if (rq->rq_flags & RQF_MQ_INFLIGHT)
> +			active++;

Are there any cases where either none or all of requests have the flag set, and
hence active == nr_tags?

Apart from that, the patch looks conceptually good.

-- 
Jens Axboe