linux-kernel - Re: [PATCH v4 1/6] nvme: multipath: Implemented new iopolicy "queue-depth"

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Date: Mon, 20 May 2024 08:46:21 -0600
From: Keith Busch <kbusch@...nel.org>
To: John Meneghini <jmeneghi@...hat.com>
Cc: tj@...nel.org, josef@...icpanda.com, axboe@...nel.dk, hch@....de,
	sagi@...mberg.me, emilne@...hat.com, hare@...nel.org,
	linux-block@...r.kernel.org, cgroups@...r.kernel.org,
	linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
	jrani@...estorage.com, randyj@...estorage.com
Subject: Re: [PATCH v4 1/6] nvme: multipath: Implemented new iopolicy
 "queue-depth"

On Tue, May 14, 2024 at 01:53:17PM -0400, John Meneghini wrote:
> @@ -130,6 +133,7 @@ void nvme_mpath_start_request(struct request *rq)
>  	if (!blk_queue_io_stat(disk->queue) || blk_rq_is_passthrough(rq))
>  		return;
>  
> +	atomic_inc(&ns->ctrl->nr_active);

Why skip passthrough and stats?

And I think you should squash the follow up patch that constrains the
atomics to the queue-depth path selector.

> +static struct nvme_ns *nvme_queue_depth_path(struct nvme_ns_head *head)
> +{
> +	struct nvme_ns *best_opt = NULL, *best_nonopt = NULL, *ns;
> +	unsigned int min_depth_opt = UINT_MAX, min_depth_nonopt = UINT_MAX;
> +	unsigned int depth;
> +
> +	list_for_each_entry_rcu(ns, &head->list, siblings) {
> +		if (nvme_path_is_disabled(ns))
> +			continue;
> +
> +		depth = atomic_read(&ns->ctrl->nr_active);
> +
> +		switch (ns->ana_state) {
> +		case NVME_ANA_OPTIMIZED:
> +			if (depth < min_depth_opt) {
> +				min_depth_opt = depth;
> +				best_opt = ns;
> +			}
> +			break;
> +
> +		case NVME_ANA_NONOPTIMIZED:
> +			if (depth < min_depth_nonopt) {
> +				min_depth_nonopt = depth;
> +				best_nonopt = ns;
> +			}
> +			break;
> +		default:
> +			break;
> +		}
> +	}
> +

I think you can do the atomic_inc here so you don't have to check the io
policy a 2nd time.

> +	return best_opt ? best_opt : best_nonopt;
> +}