Re: [PATCH V8 08/11] block: add blk_end_flush_machinery

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

On Fri, Apr 24, 2020 at 03:47:28PM +0200, Hannes Reinecke wrote:
> On 4/24/20 12:23 PM, Ming Lei wrote:
> > Flush requests aren't same with normal FS request:
> > 
> > 1) one dedicated per-hctx flush rq is pre-allocated for sending flush request
> > 
> > 2) flush request si issued to hardware via one machinary so that flush merge
> > can be applied
> > 
> > We can't simply re-submit flush rqs via blk_steal_bios(), so add
> > blk_end_flush_machinery to collect flush requests which needs to
> > be resubmitted:
> > 
> > - if one flush command without DATA is enough, send one flush, complete this
> > kind of requests
> > 
> > - otherwise, add the request into a list and let caller re-submit it.
> > 
> > Cc: John Garry <john.garry@huawei.com>
> > Cc: Bart Van Assche <bvanassche@acm.org>
> > Cc: Hannes Reinecke <hare@suse.com>
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >   block/blk-flush.c | 123 +++++++++++++++++++++++++++++++++++++++++++---
> >   block/blk.h       |   4 ++
> >   2 files changed, 120 insertions(+), 7 deletions(-)
> > 
> > diff --git a/block/blk-flush.c b/block/blk-flush.c
> > index 977edf95d711..745d878697ed 100644
> > --- a/block/blk-flush.c
> > +++ b/block/blk-flush.c
> > @@ -170,10 +170,11 @@ static void blk_flush_complete_seq(struct request *rq,
> >   	unsigned int cmd_flags;
> >   	BUG_ON(rq->flush.seq & seq);
> > -	rq->flush.seq |= seq;
> > +	if (!error)
> > +		rq->flush.seq |= seq;
> >   	cmd_flags = rq->cmd_flags;
> > -	if (likely(!error))
> > +	if (likely(!error && !fq->flush_queue_terminating))
> >   		seq = blk_flush_cur_seq(rq);
> >   	else
> >   		seq = REQ_FSEQ_DONE;
> > @@ -200,9 +201,15 @@ static void blk_flush_complete_seq(struct request *rq,
> >   		 * normal completion and end it.
> >   		 */
> >   		BUG_ON(!list_empty(&rq->queuelist));
> > -		list_del_init(&rq->flush.list);
> > -		blk_flush_restore_request(rq);
> > -		blk_mq_end_request(rq, error);
> > +
> > +		/* Terminating code will end the request from flush queue */
> > +		if (likely(!fq->flush_queue_terminating)) {
> > +			list_del_init(&rq->flush.list);
> > +			blk_flush_restore_request(rq);
> > +			blk_mq_end_request(rq, error);
> > +		} else {
> > +			list_move_tail(&rq->flush.list, pending);
> > +		}
> >   		break;
> >   	default:
> > @@ -279,7 +286,8 @@ static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
> >   	struct request *flush_rq = fq->flush_rq;
> >   	/* C1 described at the top of this file */
> > -	if (fq->flush_pending_idx != fq->flush_running_idx || list_empty(pending))
> > +	if (fq->flush_pending_idx != fq->flush_running_idx ||
> > +			list_empty(pending) || fq->flush_queue_terminating)
> >   		return;
> >   	/* C2 and C3
> > @@ -331,7 +339,7 @@ static void mq_flush_data_end_io(struct request *rq, blk_status_t error)
> >   	struct blk_flush_queue *fq = blk_get_flush_queue(q, ctx);
> >   	if (q->elevator) {
> > -		WARN_ON(rq->tag < 0);
> > +		WARN_ON(rq->tag < 0 && !fq->flush_queue_terminating);
> >   		blk_mq_put_driver_tag(rq);
> >   	}
> > @@ -503,3 +511,104 @@ void blk_free_flush_queue(struct blk_flush_queue *fq)
> >   	kfree(fq->flush_rq);
> >   	kfree(fq);
> >   }
> > +
> > +static void __blk_end_queued_flush(struct blk_flush_queue *fq,
> > +		unsigned int queue_idx, struct list_head *resubmit_list,
> > +		struct list_head *flush_list)
> > +{
> > +	struct list_head *queue = &fq->flush_queue[queue_idx];
> > +	struct request *rq, *nxt;
> > +
> > +	list_for_each_entry_safe(rq, nxt, queue, flush.list) {
> > +		unsigned int seq = blk_flush_cur_seq(rq);
> > +
> > +		list_del_init(&rq->flush.list);
> > +		blk_flush_restore_request(rq);
> > +		if (!blk_rq_sectors(rq) || seq == REQ_FSEQ_POSTFLUSH )
> > +			list_add_tail(&rq->queuelist, flush_list);
> > +		else
> > +			list_add_tail(&rq->queuelist, resubmit_list);
> > +	}
> > +}
> > +
> > +static void blk_end_queued_flush(struct blk_flush_queue *fq,
> > +		struct list_head *resubmit_list, struct list_head *flush_list)
> > +{
> > +	unsigned long flags;
> > +
> > +	spin_lock_irqsave(&fq->mq_flush_lock, flags);
> > +	__blk_end_queued_flush(fq, 0, resubmit_list, flush_list);
> > +	__blk_end_queued_flush(fq, 1, resubmit_list, flush_list);
> > +	spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
> > +}
> > +
> > +/* complete requests which just requires one flush command */
> > +static void blk_complete_flush_requests(struct blk_flush_queue *fq,
> > +		struct list_head *flush_list)
> > +{
> > +	struct block_device *bdev;
> > +	struct request *rq;
> > +	int error = -ENXIO;
> > +
> > +	if (list_empty(flush_list))
> > +		return;
> > +
> > +	rq = list_first_entry(flush_list, struct request, queuelist);
> > +
> > +	/* Send flush via one active hctx so we can move on */
> > +	bdev = bdget_disk(rq->rq_disk, 0);
> > +	if (bdev) {
> > +		error = blkdev_issue_flush(bdev, GFP_KERNEL, NULL);
> > +		bdput(bdev);
> > +	}
> > +
> > +	while (!list_empty(flush_list)) {
> > +		rq = list_first_entry(flush_list, struct request, queuelist);
> > +		list_del_init(&rq->queuelist);
> > +		blk_mq_end_request(rq, error);
> > +	}
> > +}
> > +
> I must admit I'm getting nervous if one mixes direct request manipulations
> with high-level 'blkdev_XXX' calls.
> Can't we just requeue everything including the flush itself, and letting the
> requeue algorithm pick a new hctx?

No, the biggest problem is that rq->mq_hctx is bound to the hctx(becomes inactive now)
since request allocation, so we can't requeue them simply.

What we need here is simply one flush command, blkdev_issue_flush() does
exactly what we want.

Thanks,
Ming