Re: [PATCH v5] nvme: multipath: Implemented new iopolicy "queue-depth"

[Chaitanya Kulkarni]

On 5/22/24 09:54, John Meneghini wrote:
> From: "Ewan D. Milne" <emilne@xxxxxxxxxx>
>
> The round-robin path selector is inefficient in cases where there is a
> difference in latency between paths.  In the presence of one or more
> high latency paths the round-robin selector continues to use the high
> latency path equally. This results in a bias towards the highest latency
> path and can cause a significant decrease in overall performance as IOs
> pile on the highest latency path. This problem is acute with NVMe-oF
> controllers.
>
> The queue-depth policy instead sends I/O requests down the path with the
> least amount of requests in its request queue. Paths with lower latency
> will clear requests more quickly and have less requests in their queues
> compared to higher latency paths. The goal of this path selector is to
> make more use of lower latency paths which will bring down overall IO
> latency and increase throughput and performance.
>
> Signed-off-by: Thomas Song <tsong@xxxxxxxxxxxxxxx>
> [emilne: patch developed by Thomas Song @ Pure Storage, fixed whitespace
>        and compilation warnings, updated MODULE_PARM description, and
>        fixed potential issue with ->current_path[] being used]
> Signed-off-by: Ewan D. Milne <emilne@xxxxxxxxxx>
> [jmeneghi: vairious changes and improvements, addressed review comments]
> Signed-off-by: John Meneghini <jmeneghi@xxxxxxxxxx>
> Link: https://lore.kernel.org/linux-nvme/20240509202929.831680-1-jmeneghi@xxxxxxxxxx/
> Tested-by: Marco Patalano <mpatalan@xxxxxxxxxx>
> Reviewed-by: Randy Jennings <randyj@xxxxxxxxxxxxxxx>
> Tested-by: Jyoti Rani <jrani@xxxxxxxxxxxxxxx>
> ---

[...]

> +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;
> +

nit:- no need to add white line needed after break above ?

> +		case NVME_ANA_NONOPTIMIZED:
> +			if (depth < min_depth_nonopt) {
> +				min_depth_nonopt = depth;
> +				best_nonopt = ns;
> +			}
> +			break;
> +		default:
> +			break;
> +		}
> +
> +		if (min_depth_opt == 0)
> +			return best_opt;
> +	}
> +
> +	return best_opt ? best_opt : best_nonopt;
> +}
> +
>   

[...]

> @@ -800,6 +860,29 @@ static ssize_t nvme_subsys_iopolicy_show(struct device *dev,
>   			  nvme_iopolicy_names[READ_ONCE(subsys->iopolicy)]);
>   }
>   
> +static void nvme_subsys_iopolicy_update(struct nvme_subsystem *subsys, int iopolicy)

nit:- overly long line, as rest of the file is < 80 char par line ?

> +{
> +	struct nvme_ctrl *ctrl;
> +	int old_iopolicy = READ_ONCE(subsys->iopolicy);
> +
> +	if (old_iopolicy == iopolicy)
> +		return;
> +
> +	WRITE_ONCE(subsys->iopolicy, iopolicy);
> +
> +	/* iopolicy changes reset the counters and clear the mpath by design */
> +	mutex_lock(&nvme_subsystems_lock);
> +	list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry) {
> +		atomic_set(&ctrl->nr_active, 0);
> +		nvme_mpath_clear_ctrl_paths(ctrl);
> +	}
> +	mutex_unlock(&nvme_subsystems_lock);
> +
> +	pr_notice("%s: changed from %s to %s for subsysnqn %s\n", __func__,
> +			nvme_iopolicy_names[old_iopolicy], nvme_iopolicy_names[iopolicy],

nit: overly long line above as rest of the file is < 80 char ...

As far as I remember, most of the nvme code uses pr_info(), but if
decision has been made to use pr_notice() for a specific reason then
please ignore this comment.

> +			subsys->subnqn);
> +}
> +

[...]

> diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
> index fc31bd340a63..fa3833d88a85 100644
> --- a/drivers/nvme/host/nvme.h
> +++ b/drivers/nvme/host/nvme.h
> @@ -50,6 +50,8 @@ extern struct workqueue_struct *nvme_wq;
>   extern struct workqueue_struct *nvme_reset_wq;
>   extern struct workqueue_struct *nvme_delete_wq;
>   
> +extern struct mutex nvme_subsystems_lock;
> +
>   /*
>    * List of workarounds for devices that required behavior not specified in
>    * the standard.
> @@ -195,6 +197,7 @@ enum {
>   	NVME_REQ_CANCELLED		= (1 << 0),
>   	NVME_REQ_USERCMD		= (1 << 1),
>   	NVME_MPATH_IO_STATS		= (1 << 2),
> +	NVME_MPATH_CNT_ACTIVE	= (1 << 3),

nit:- I think we need to align above line to rest of the members in
       this enum ...

>   };
>   
>   static inline struct nvme_request *nvme_req(struct request *req)
> @@ -359,6 +362,7 @@ struct nvme_ctrl {
>   	size_t ana_log_size;
>   	struct timer_list anatt_timer;
>   	struct work_struct ana_work;
> +	atomic_t nr_active;
>   #endif
>   
>   #ifdef CONFIG_NVME_HOST_AUTH
> @@ -407,6 +411,7 @@ static inline enum nvme_ctrl_state nvme_ctrl_state(struct nvme_ctrl *ctrl)
>   enum nvme_iopolicy {
>   	NVME_IOPOLICY_NUMA,
>   	NVME_IOPOLICY_RR,
> +	NVME_IOPOLICY_QD,
>   };
>   
>   struct nvme_subsystem {

apart from the few nits patch does looks good to me.

Reviewed-by: Chaitanya Kulkarni <kch@xxxxxxxxxx>

not a blocker to merge this patch, but we need a blktests for this code 
in nvme
category ...

-ck