[PATCH v2 2/2] nvme: support io stats on the mpath device

[Keith Busch]

On Mon, Oct 03, 2022 at 12:43:44PM +0300, Sagi Grimberg wrote:
> Our mpath stack device is just a shim that selects a bottom namespace
> and submits the bio to it without any fancy splitting. This also means
> that we don't clone the bio or have any context to the bio beyond
> submission. However it really sucks that we don't see the mpath device
> io stats.
> 
> Given that the mpath device can't do that without adding some context
> to it, we let the bottom device do it on its behalf (somewhat similar
> to the approach taken in nvme_trace_bio_complete).
> 
> When the IO starts, we account the request for multipath IO stats using
> REQ_NVME_MPATH_IO_STATS nvme_request flag to avoid queue io stats disable
> in the middle of the request.

An unfortunate side effect is that a successful error failover will get
accounted for twice in the mpath device, but cloning to create a
separate context just to track iostats for that unusual condition is
much worse.

Reviewed-by: Keith Busch <kbusch at kernel.org>

> +void nvme_mpath_start_request(struct request *rq)
> +{
> +	struct nvme_ns *ns = rq->q->queuedata;
> +	struct gendisk *disk = ns->head->disk;
> +
> +	if (!blk_queue_io_stat(disk->queue) || blk_rq_is_passthrough(rq))
> +		return;
> +
> +	nvme_req(rq)->flags |= NVME_MPATH_IO_STATS;
> +	nvme_req(rq)->start_time = bdev_start_io_acct(disk->part0,
> +					blk_rq_bytes(rq) >> SECTOR_SHIFT,
> +					req_op(rq), jiffies);
> +}
> +void nvme_mpath_end_request(struct request *rq)
> +{
> +	struct nvme_ns *ns = rq->q->queuedata;
> +
> +	if (!(nvme_req(rq)->flags & NVME_MPATH_IO_STATS))
> +		return;
> +	bdev_end_io_acct(ns->head->disk->part0, req_op(rq),
> +		nvme_req(rq)->start_time);
> +}

I think these also can be static inline.