[PATCH] nvme: Update node paths after adding new path

[keith.busch@intel.com (Keith Busch)]

On Fri, Oct 05, 2018@11:10:59AM -0700, Sagi Grimberg wrote:
> 
> > The nvme namespace paths were being updated only when the current path
> > was not set or nonoptimized. If a new path comes online that is a better
> > path for its NUMA node, the multipath selector may continue using the
> > previously set path on a potentially further node.
> > 
> > This patch re-runs the path assignment after successfully adding a new
> > optimized path.
> > 
> > Signed-off-by: Keith Busch <keith.busch at intel.com>
> > ---
> >   drivers/nvme/host/multipath.c | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> > 
> > diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> > index 52987052b7fc..fc3c5c2b3eca 100644
> > --- a/drivers/nvme/host/multipath.c
> > +++ b/drivers/nvme/host/multipath.c
> > @@ -321,6 +321,14 @@ static void nvme_mpath_set_live(struct nvme_ns *ns)
> >   		device_add_disk(&head->subsys->dev, head->disk,
> >   				nvme_ns_id_attr_groups);
> > +	if (nvme_path_is_optimized(ns)) {
> > +		int node, srcu_idx;
> > +
> > +		srcu_idx = srcu_read_lock(&head->srcu);
> > +		for_each_node(node)
> > +			__nvme_find_path(head, node);
> > +		srcu_read_unlock(&head->srcu, srcu_idx);
> > +	}
> 
> Any reason for interfering with all the numa nodes?
> 
> Why not simple call __nvme_find_path(head, ns->ctrl->node_id) ?

The new path's local node may be closer to other nodes who's path is
currently set to something else.

Consider the following simplified example of nodes A through D:

  -----    -----    -----    ----- 
  | A |<-->| B |<-->| C |<-->| D |
  -----    -----    -----    -----

Let's say we start with a single optimized path that is local to node
"D". That path will initially be the "current_path" for all nodes.

Now attach a new path local to node "A". Updating just node A's
current_path misses updating B away from D, even though A is closer
than D.