Re: [PATCHv4 RFC 1/1] nvme-multipath: Add sysfs attributes for showing multipath info

[Hannes Reinecke <hare@suse.de>]

On 10/7/24 15:47, Nilay Shroff wrote:
> 
> 
> On 10/7/24 15:44, Hannes Reinecke wrote:
>> On 9/11/24 08:26, Nilay Shroff wrote:
>>> NVMe native multipath supports different IO policies for selecting I/O
>>> path, however we don't have any visibility about which path is being
>>> selected by multipath code for forwarding I/O.
>>> This patch helps add that visibility by adding new sysfs attribute files
>>> named "numa_nodes" and "queue_depth" under each namespace block device
>>> path /sys/block/nvmeXcYnZ/. We also create a "multipath" sysfs directory
>>> under head disk node and then from this directory add a link to each
>>> namespace path device this head disk node points to.
>>>
>>> For instance, /sys/block/nvmeXnY/multipath/ would create a soft link to
>>> each path the head disk node <nvmeXnY> points to:
>>>
>>> $ ls -1 /sys/block/nvme1n1/
>>> nvme1c1n1 -> ../../../../../pci052e:78/052e:78:00.0/nvme/nvme1/nvme1c1n1
>>> nvme1c3n1 -> ../../../../../pci058e:78/058e:78:00.0/nvme/nvme3/nvme1c3n1
>>>
>>> For round-robin I/O policy, we could easily infer from the above output
>>> that I/O workload targeted to nvme3n1 would toggle across paths nvme1c1n1
>>> and nvme1c3n1.
>>>
>>> For numa I/O policy, the "numa_nodes" attribute file shows the numa nodes
>>> being preferred by the respective block device path. The numa nodes value
>>> is comma delimited list of nodes or A-B range of nodes.
>>>
>>> For queue-depth I/O policy, the "queue_depth" attribute file shows the
>>> number of active/in-flight I/O requests currently queued for each path.
>>>
>>> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
>>> ---
>>>    drivers/nvme/host/core.c      |  3 ++
>>>    drivers/nvme/host/multipath.c | 71 +++++++++++++++++++++++++++++++++++
>>>    drivers/nvme/host/nvme.h      | 20 ++++++++--
>>>    drivers/nvme/host/sysfs.c     | 20 ++++++++++
>>>    4 files changed, 110 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
>>> index 983909a600ad..6be29fd64236 100644
>>> --- a/drivers/nvme/host/core.c
>>> +++ b/drivers/nvme/host/core.c
>>> @@ -3951,6 +3951,9 @@ static void nvme_ns_remove(struct nvme_ns *ns)
>>>          if (!nvme_ns_head_multipath(ns->head))
>>>            nvme_cdev_del(&ns->cdev, &ns->cdev_device);
>>> +
>>> +    nvme_mpath_remove_sysfs_link(ns);
>>> +
>>>        del_gendisk(ns->disk);
>>>          mutex_lock(&ns->ctrl->namespaces_lock);
>>> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
>>> index 518e22dd4f9b..7d9c36a7a261 100644
>>> --- a/drivers/nvme/host/multipath.c
>>> +++ b/drivers/nvme/host/multipath.c
>>> @@ -654,6 +654,8 @@ static void nvme_mpath_set_live(struct nvme_ns *ns)
>>>            nvme_add_ns_head_cdev(head);
>>>        }
>>>    +    nvme_mpath_add_sysfs_link(ns);
>>> +
>>>        mutex_lock(&head->lock);
>>>        if (nvme_path_is_optimized(ns)) {
>>>            int node, srcu_idx;
>> Nearly there.
> Thank you for your review comments!
> 
>>
>> You can only call 'nvme_mpath_add_sysfs_link()' if the gendisk on the head had been created.
>>
>> And there is one branch in nvme_mpath_add_disk():
>>
>>                  if (desc.state) {
>>                          /* found the group desc: update */
>>                          nvme_update_ns_ana_state(&desc, ns);
>>
>> which does not go via nvme_mpath_set_live(), yet a device link would need to be create here, too.
>> But you can't call nvme_mpath_add_sysfs_link() from nvme_mpath_add_disk(), as the actual gendisk
 >> might only be created later on during ANA log parsing.>>
>> It is a tangle, and I haven't found a good way out of this.
>> But I am _very much_ in favour of having these links, so please
>> update your patch.
>> In case disk supports ANA group then yes it would go through nvme_mpath_add_disk()->nvme_update_ns_ana_state();
>> and later nvme_update_ns_ana_state() would also fall through function nvme_mpath_set_live where we call
>> nvme_mpath_add_sysfs_link().
> 
> So I think that in any case while multipath namespace is being created it has to go through
> nvme_mpath_set_live function. And as we see in nvme_mpath_set_live function, we only create
> sysfs link after the gendisk on the head is created. Do you agree with this? Or please let
> me know if you have any further question.
> 
But it doesn't:

         if (nvme_state_is_live(ns->ana_state) &&
             nvme_ctrl_state(ns->ctrl) == NVME_CTRL_LIVE)
                 nvme_mpath_set_live(ns);

so if a namespace is being created for which nvme_state_is_live() 
returns 'false' nvme_mpath_set_live() is not called, and no links are 
being created.
This can happen eg. if the first namespace to be encountered is in any 
other state than 'optimized' or 'non-optimized'.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
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