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

[Nilay Shroff]

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?
> 
>> Cheers,
>>
>> Hannes
>
A gentle ping about this thread. Does change look okay or you have further comments?
Am I missing something here? If so, can you please help me with it?

Thanks,
--Nilay