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

[Ming Lei <ming.lei@redhat.com>]

On Tue, Nov 02, 2021 at 08:57:41AM -0600, Jens Axboe wrote:
> On 11/2/21 7:57 AM, Ming Lei wrote:
> > On Tue, Nov 02, 2021 at 07:47:44AM -0600, Jens Axboe wrote:
> >> 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?
> > 
> > none and BLK_MQ_F_TAG_QUEUE_SHARED, and Shinichiro only observed the
> > issue on two NSs.
> 
> Maybe I wasn't clear enough. What I'm saying is that either all of the
> requests will have RQF_MQ_INFLIGHT set, or none of them. Hence active
> should be either 0, or == nr_tags.

Yeah, that is right since BLK_MQ_F_TAG_QUEUE_SHARED is updated after
queue is frozen. Meantime blk_mq_end_request_batch() is only called
for ending successfully completed requests.

Will do that in V2.

Thanks,
Ming