[PATCH] nvme-multipath: implement active-active round-robin path selector

[Keith Busch]

On Wed, Mar 28, 2018 at 10:06:46AM +0200, Christoph Hellwig wrote:
> For PCIe devices the right policy is not a round robin but to use
> the pcie device closer to the node.  I did a prototype for that
> long ago and the concept can work.  Can you look into that and
> also make that policy used automatically for PCIe devices?

Yeah, that is especially true if you've multiple storage accessing
threads scheduled on different nodes. On the other hand, round-robin
may still benefit if both paths are connected to different root ports
on the same node (who would do that?!).

But I wasn't aware people use dual-ported PCIe NVMe connected to a
single host (single path from two hosts seems more common). If that's a
thing, we should get some numa awareness. I couldn't find your prototype,
though. I had one stashed locally from a while back and hope it resembles
what you had in mind:
---
struct nvme_ns *nvme_find_path_numa(struct nvme_ns_head *head)
{
        int distance, current = INT_MAX, node = cpu_to_node(smp_processor_id());
        struct nvme_ns *ns, *path = NULL;

        list_for_each_entry_rcu(ns, &head->list, siblings) {
                if (ns->ctrl->state != NVME_CTRL_LIVE)
                        continue;
                if (ns->disk->node_id == node)
                        return ns;

                distance = node_distance(node, ns->disk->node_id);
                if (distance < current) {
                        current = distance;
                        path = ns;
                }
        }
        return path;
}
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