Re: [PATCH v6 1/1] nvme-multipath: implement "queue-depth" iopolicy

[Hannes Reinecke <hare@suse.de>]

On 6/12/24 02:20, John Meneghini wrote:
> From: Thomas Song <tsong@purestorage.com>
> 
> 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@purestorage.com>
> [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]
> Co-developed-by: Ewan D. Milne <emilne@redhat.com>
> Signed-off-by: Ewan D. Milne <emilne@redhat.com>
> [jmeneghi: vairious changes and improvements, addressed review comments]
> Co-developed-by: John Meneghini <jmeneghi@redhat.com>
> Signed-off-by: John Meneghini <jmeneghi@redhat.com>
> Link: https://lore.kernel.org/linux-nvme/20240509202929.831680-1-jmeneghi@redhat.com/
> Tested-by: Marco Patalano <mpatalan@redhat.com>
> Reviewed-by: Randy Jennings <randyj@purestorage.com>
> Tested-by: Jyoti Rani <jrani@purestorage.com>
> ---
>   drivers/nvme/host/core.c      |   2 +-
>   drivers/nvme/host/multipath.c | 108 +++++++++++++++++++++++++++++++---
>   drivers/nvme/host/nvme.h      |   5 ++
>   3 files changed, 106 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index 7c9f91314d36..c10ff8815d82 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -110,7 +110,7 @@ struct workqueue_struct *nvme_delete_wq;
>   EXPORT_SYMBOL_GPL(nvme_delete_wq);
>   
>   static LIST_HEAD(nvme_subsystems);
> -static DEFINE_MUTEX(nvme_subsystems_lock);
> +DEFINE_MUTEX(nvme_subsystems_lock);
>   
>   static DEFINE_IDA(nvme_instance_ida);
>   static dev_t nvme_ctrl_base_chr_devt;
> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> index 03a6868f4dbc..fe10e0cebcf0 100644
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -17,6 +17,7 @@ MODULE_PARM_DESC(multipath,
>   static const char *nvme_iopolicy_names[] = {
>   	[NVME_IOPOLICY_NUMA]	= "numa",
>   	[NVME_IOPOLICY_RR]	= "round-robin",
> +	[NVME_IOPOLICY_QD]      = "queue-depth",
>   };
>   
>   static int iopolicy = NVME_IOPOLICY_NUMA;
> @@ -29,6 +30,8 @@ static int nvme_set_iopolicy(const char *val, const struct kernel_param *kp)
>   		iopolicy = NVME_IOPOLICY_NUMA;
>   	else if (!strncmp(val, "round-robin", 11))
>   		iopolicy = NVME_IOPOLICY_RR;
> +	else if (!strncmp(val, "queue-depth", 11))
> +		iopolicy = NVME_IOPOLICY_QD;
>   	else
>   		return -EINVAL;
>   
> @@ -43,7 +46,7 @@ static int nvme_get_iopolicy(char *buf, const struct kernel_param *kp)
>   module_param_call(iopolicy, nvme_set_iopolicy, nvme_get_iopolicy,
>   	&iopolicy, 0644);
>   MODULE_PARM_DESC(iopolicy,
> -	"Default multipath I/O policy; 'numa' (default) or 'round-robin'");
> +	"Default multipath I/O policy; 'numa' (default), 'round-robin' or 'queue-depth'");
>   
>   void nvme_mpath_default_iopolicy(struct nvme_subsystem *subsys)
>   {
> @@ -128,6 +131,11 @@ void nvme_mpath_start_request(struct request *rq)
>   	struct nvme_ns *ns = rq->q->queuedata;
>   	struct gendisk *disk = ns->head->disk;
>   
> +	if (READ_ONCE(ns->head->subsys->iopolicy) == NVME_IOPOLICY_QD) {
> +		atomic_inc(&ns->ctrl->nr_active);
> +		nvme_req(rq)->flags |= NVME_MPATH_CNT_ACTIVE;
> +	}
> +
>   	if (!blk_queue_io_stat(disk->queue) || blk_rq_is_passthrough(rq))
>   		return;
>   
> @@ -140,6 +148,12 @@ EXPORT_SYMBOL_GPL(nvme_mpath_start_request);
>   void nvme_mpath_end_request(struct request *rq)
>   {
>   	struct nvme_ns *ns = rq->q->queuedata;
> +	int result;
> +
> +	if ((nvme_req(rq)->flags & NVME_MPATH_CNT_ACTIVE)) {
> +		result = atomic_dec_if_positive(&ns->ctrl->nr_active);
> +		WARN_ON_ONCE(result < 0);
> +	}
>   
>   	if (!(nvme_req(rq)->flags & NVME_MPATH_IO_STATS))
>   		return;
> @@ -291,10 +305,15 @@ static struct nvme_ns *nvme_next_ns(struct nvme_ns_head *head,
>   	return list_first_or_null_rcu(&head->list, struct nvme_ns, siblings);
>   }
>   
> -static struct nvme_ns *nvme_round_robin_path(struct nvme_ns_head *head,
> -		int node, struct nvme_ns *old)
> +static struct nvme_ns *nvme_round_robin_path(struct nvme_ns_head *head)
>   {
> -	struct nvme_ns *ns, *found = NULL;
> +	struct nvme_ns *ns, *old, *found = NULL;
> +	int node = numa_node_id();
> +
> +	old = srcu_dereference(head->current_path[node], &head->srcu);
> +
> +	if (unlikely(!old))
> +		return __nvme_find_path(head, node);
>   
>   	if (list_is_singular(&head->list)) {
>   		if (nvme_path_is_disabled(old))
> @@ -334,13 +353,49 @@ static struct nvme_ns *nvme_round_robin_path(struct nvme_ns_head *head,
>   	return found;
>   }
>   
> +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;
> +		}
> +
> +		if (min_depth_opt == 0)
> +			return best_opt;
> +	}
> +
> +	return best_opt ? best_opt : best_nonopt;
> +}
> +
>   static inline bool nvme_path_is_optimized(struct nvme_ns *ns)
>   {
>   	return nvme_ctrl_state(ns->ctrl) == NVME_CTRL_LIVE &&
>   		ns->ana_state == NVME_ANA_OPTIMIZED;
>   }
>   
> -inline struct nvme_ns *nvme_find_path(struct nvme_ns_head *head)
> +static struct nvme_ns *nvme_numa_path(struct nvme_ns_head *head)
>   {
>   	int node = numa_node_id();
>   	struct nvme_ns *ns;
> @@ -349,13 +404,25 @@ inline struct nvme_ns *nvme_find_path(struct nvme_ns_head *head)
>   	if (unlikely(!ns))
>   		return __nvme_find_path(head, node);
>   
> -	if (READ_ONCE(head->subsys->iopolicy) == NVME_IOPOLICY_RR)
> -		return nvme_round_robin_path(head, node, ns);
>   	if (unlikely(!nvme_path_is_optimized(ns)))
>   		return __nvme_find_path(head, node);
> +

Pointless newline.

But other than that:

Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
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