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

[Hannes Reinecke]

On 10/16/24 05:19, Nilay Shroff wrote:
> Hi Hannes,
> 
> On 10/7/24 21:03, Nilay Shroff wrote:
>>
>>
>> On 10/7/24 19:34, Hannes Reinecke wrote:
>>> 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 at 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'.
>>>
>>
>> OK I got what you're suggesting here. So in this particular case when ANA state of a shared namespace
>> is neither "optimized" nor "non-optimized", we would have gendisk for shared namepspace (i.e.
>> nvmeXcYnZ) created but we don't have yet gendisk for corresponding head node (i.e. nvmeXnY) created.
>> So without the gendisk for head node created, how could we create a link from it to the namespace node?
>>
>> The link from gendisk head node would be eventually created when the ANA state of the namespace transition
>> to "optimized" or "non-optimized" state. I think, it's not anyways possible to have multipathing function
>> enabled until the gendisk for the head node created, isn't it? So I don't yet understand why do we really
>> need device link created if we don't have gendisk for head not ready? Am I missing anything here?
>>
You are right, we will create 'the' link for the gendisk node once 
'nvme_mpath_set_live()' is called. But:
Then we would have _one_ link (for the active path) only.
All other paths (ie the non-active ones) might already been present, but 
won't get a link.

I'd love to have links to all paths present, active or not.

Cheers,

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