On Thu, 2018-01-11 at 14:01 +0800, Ming Lei wrote:
> diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
> index 86bf502a8e51..fcddf5a62581 100644
> --- a/drivers/md/dm-mpath.c
> +++ b/drivers/md/dm-mpath.c
> @@ -533,8 +533,20 @@ static int multipath_clone_and_map(struct dm_target *ti, struct request *rq,
> if (queue_dying) {
> atomic_inc(&m->pg_init_in_progress);
> activate_or_offline_path(pgpath);
> + return DM_MAPIO_DELAY_REQUEUE;
> }
> - return DM_MAPIO_DELAY_REQUEUE;
> +
> + /*
> + * blk-mq's SCHED_RESTART can cover this requeue, so
> + * we needn't to deal with it by DELAY_REQUEUE. More
> + * importantly, we have to return DM_MAPIO_REQUEUE
> + * so that blk-mq can get the queue busy feedback,
> + * otherwise I/O merge can be hurt.
> + */
> + if (q->mq_ops)
> + return DM_MAPIO_REQUEUE;
> + else
> + return DM_MAPIO_DELAY_REQUEUE;
> }
Sorry but the approach of this patch looks wrong to me. I'm afraid that this
approach will cause 100% CPU consumption if the underlying .queue_rq()
function returns BLK_STS_RESOURCE for another reason than what can be detected
by the .get_budget() call. This can happen if e.g. a SCSI LLD .queuecommand()
implementation returns SCSI_MLQUEUE_HOST_BUSY. Many SCSI LLDs can do this:
$ git grep 'SCSI_MLQUEUE_[^_]*_BUSY' | wc -l
204
Isn't this a severe regression?
Bart.
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