Re: [PATCHv2 2/4] tree: add queue-depth attribute for nvme path object

[Nilay Shroff <nilay@linux.ibm.com>]

On 4/22/25 11:56 AM, Hannes Reinecke wrote:
> On 4/17/25 15:59, Nilay Shroff wrote:
>> Add a new attribute named "queue_depth" under the NVMe path object. This
>> attribute is used by the iopolicy "queue-depth", which was introduced in
>> kernel v6.11. However, the corresponding sysfs attribute for queue depth
>> was only added in kernel v6.14.
>>
>> The queue_depth value can be useful for observing which paths are selected
>> for I/O forwarding, based on the depth of each path. To support this,
>> export the attribute in libnvme.map so it can be accessed via nvme-cli.
>>
>> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
>> ---
>>   src/libnvme.map    |  1 +
>>   src/nvme/private.h |  1 +
>>   src/nvme/tree.c    | 12 +++++++++++-
>>   src/nvme/tree.h    |  8 ++++++++
>>   4 files changed, 21 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/libnvme.map b/src/libnvme.map
>> index 4314705f..e53fad6b 100644
>> --- a/src/libnvme.map
>> +++ b/src/libnvme.map
>> @@ -317,6 +317,7 @@ LIBNVME_1_0 {
>>           nvme_path_get_ctrl;
>>           nvme_path_get_name;
>>           nvme_path_get_ns;
>> +        nvme_path_get_queue_depth;
>>           nvme_path_get_sysfs_dir;
>>           nvme_paths_filter;
>>           nvme_read_config;
>> diff --git a/src/nvme/private.h b/src/nvme/private.h
>> index f45c5823..f94276e2 100644
>> --- a/src/nvme/private.h
>> +++ b/src/nvme/private.h
>> @@ -34,6 +34,7 @@ struct nvme_path {
>>       char *sysfs_dir;
>>       char *ana_state;
>>       int grpid;
>> +    int queue_depth;
>>   };
>>     struct nvme_ns_head {
>> diff --git a/src/nvme/tree.c b/src/nvme/tree.c
>> index bd7fb53e..b7a38a07 100644
>> --- a/src/nvme/tree.c
>> +++ b/src/nvme/tree.c
>> @@ -903,6 +903,11 @@ const char *nvme_path_get_name(nvme_path_t p)
>>       return p->name;
>>   }
>>   +int nvme_path_get_queue_depth(nvme_path_t p)
>> +{
>> +    return p->queue_depth;
>> +}
>> +
>>   const char *nvme_path_get_ana_state(nvme_path_t p)
>>   {
>>       return p->ana_state;
>> @@ -921,7 +926,7 @@ void nvme_free_path(struct nvme_path *p)
>>   static int nvme_ctrl_scan_path(nvme_root_t r, struct nvme_ctrl *c, char *name)
>>   {
>>       struct nvme_path *p;
>> -    _cleanup_free_ char *path = NULL, *grpid = NULL;
>> +    _cleanup_free_ char *path = NULL, *grpid = NULL, *queue_depth = NULL;
>>       int ret;
>>         nvme_msg(r, LOG_DEBUG, "scan controller %s path %s\n",
>> @@ -955,6 +960,11 @@ static int nvme_ctrl_scan_path(nvme_root_t r, struct nvme_ctrl *c, char *name)
>>           sscanf(grpid, "%d", &p->grpid);
>>       }
>>   +    queue_depth = nvme_get_path_attr(p, "queue_depth");
>> +    if (queue_depth) {
>> +        sscanf(queue_depth, "%d", &p->queue_depth);
>> +    }
>> +
>>       list_node_init(&p->nentry);
>>       list_node_init(&p->entry);
>>       list_add_tail(&c->paths, &p->entry);
>> diff --git a/src/nvme/tree.h b/src/nvme/tree.h
>> index 9f382e9c..a9082f8e 100644
>> --- a/src/nvme/tree.h
>> +++ b/src/nvme/tree.h
>> @@ -867,6 +867,14 @@ const char *nvme_path_get_sysfs_dir(nvme_path_t p);
>>    */
>>   const char *nvme_path_get_ana_state(nvme_path_t p);
>>   +/**
>> + * nvme_path_get_queue_depth() - Queue depth of an nvme_path_t object
>> + * @p: &nvme_path_t object
>> + *
>> + * Return: Queue depth of @p
>> + */
>> +int nvme_path_get_queue_depth(nvme_path_t p);
>> +
>>   /**
>>    * nvme_path_get_ctrl() - Parent controller of an nvme_path_t object
>>    * @p:    &nvme_path_t object
> 
> As discussed: saving the 'queue_depth' attribute is a bad idea, as it
> changes frequently.
> I'd rather read the attribute value from sysfs whenever
> 'nvme_path_get_queue_depth' is called, and use the tree structure only
> to hold the pathname.
> 
Yes correct, but I thought we discussed about implementing non-cached
version of API in libnvme 2.x. So I implemented cached version of 
queue_depth API. Daniel can you confirm? 

Thanks,
--Nilay