[PATCH v8 8/8] nvme-multipath: queue-depth support for marginal paths

[PATCH v8 8/8] nvme-multipath: queue-depth support for marginal paths

[Bryan Gurney]

From: John Meneghini <jmeneghi@redhat.com>

Exclude marginal paths from queue-depth io policy. In the case where all
paths are marginal and no optimized or non-optimized path is found, we
fall back to __nvme_find_path which selects the best marginal path.

Tested-by: Bryan Gurney <bgurney@redhat.com>
Signed-off-by: John Meneghini <jmeneghi@redhat.com>
---
 drivers/nvme/host/multipath.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 8d4e54bb4261..767583e8454b 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -420,6 +420,9 @@ static struct nvme_ns *nvme_queue_depth_path(struct nvme_ns_head *head)
 		if (nvme_path_is_disabled(ns))
 			continue;
 
+		if (nvme_ctrl_is_marginal(ns->ctrl))
+			continue;
+
 		depth = atomic_read(&ns->ctrl->nr_active);
 
 		switch (ns->ana_state) {
@@ -443,7 +446,9 @@ static struct nvme_ns *nvme_queue_depth_path(struct nvme_ns_head *head)
 			return best_opt;
 	}
 
-	return best_opt ? best_opt : best_nonopt;
+	best_opt = (best_opt) ? best_opt : best_nonopt;
+
+	return best_opt ? best_opt : __nvme_find_path(head, numa_node_id());
 }
 
 static inline bool nvme_path_is_optimized(struct nvme_ns *ns)
-- 
2.50.0

Re: [PATCH v8 8/8] nvme-multipath: queue-depth support for marginal paths

[John Meneghini]

Hannes, this patch fixes the queue-depth scheduler.  Please take a look.

On 7/9/25 5:26 PM, Bryan Gurney wrote:
> From: John Meneghini <jmeneghi@redhat.com>
> 
> Exclude marginal paths from queue-depth io policy. In the case where all
> paths are marginal and no optimized or non-optimized path is found, we
> fall back to __nvme_find_path which selects the best marginal path.
> 
> Tested-by: Bryan Gurney <bgurney@redhat.com>
> Signed-off-by: John Meneghini <jmeneghi@redhat.com>
> ---
>   drivers/nvme/host/multipath.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> index 8d4e54bb4261..767583e8454b 100644
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -420,6 +420,9 @@ static struct nvme_ns *nvme_queue_depth_path(struct nvme_ns_head *head)
>   		if (nvme_path_is_disabled(ns))
>   			continue;
>   
> +		if (nvme_ctrl_is_marginal(ns->ctrl))
> +			continue;
> +
>   		depth = atomic_read(&ns->ctrl->nr_active);
>   
>   		switch (ns->ana_state) {
> @@ -443,7 +446,9 @@ static struct nvme_ns *nvme_queue_depth_path(struct nvme_ns_head *head)
>   			return best_opt;
>   	}
>   
> -	return best_opt ? best_opt : best_nonopt;
> +	best_opt = (best_opt) ? best_opt : best_nonopt;
> +
> +	return best_opt ? best_opt : __nvme_find_path(head, numa_node_id());
>   }
>   
>   static inline bool nvme_path_is_optimized(struct nvme_ns *ns)

Re: [PATCH v8 8/8] nvme-multipath: queue-depth support for marginal paths

[Hannes Reinecke]

On 7/10/25 00:03, John Meneghini wrote:
> Hannes, this patch fixes the queue-depth scheduler.  Please take a look.
> 
> On 7/9/25 5:26 PM, Bryan Gurney wrote:
>> From: John Meneghini <jmeneghi@redhat.com>
>>
>> Exclude marginal paths from queue-depth io policy. In the case where all
>> paths are marginal and no optimized or non-optimized path is found, we
>> fall back to __nvme_find_path which selects the best marginal path.
>>
>> Tested-by: Bryan Gurney <bgurney@redhat.com>
>> Signed-off-by: John Meneghini <jmeneghi@redhat.com>
>> ---
>>   drivers/nvme/host/multipath.c | 7 ++++++-
>>   1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/ 
>> multipath.c
>> index 8d4e54bb4261..767583e8454b 100644
>> --- a/drivers/nvme/host/multipath.c
>> +++ b/drivers/nvme/host/multipath.c
>> @@ -420,6 +420,9 @@ static struct nvme_ns 
>> *nvme_queue_depth_path(struct nvme_ns_head *head)
>>           if (nvme_path_is_disabled(ns))
>>               continue;
>> +        if (nvme_ctrl_is_marginal(ns->ctrl))
>> +            continue;
>> +
>>           depth = atomic_read(&ns->ctrl->nr_active);
>>           switch (ns->ana_state) {
>> @@ -443,7 +446,9 @@ static struct nvme_ns 
>> *nvme_queue_depth_path(struct nvme_ns_head *head)
>>               return best_opt;
>>       }
>> -    return best_opt ? best_opt : best_nonopt;
>> +    best_opt = (best_opt) ? best_opt : best_nonopt;
>> +
>> +    return best_opt ? best_opt : __nvme_find_path(head, numa_node_id());
>>   }
>>   static inline bool nvme_path_is_optimized(struct nvme_ns *ns)
> 

Hmm. Not convinced. I would expect a 'marginal' path to behave different
(performance-wise) than unaffected paths. And the queue-depth scheduler
should be able to handle paths with different performance
characteristics just fine.
(Is is possible that your results are test artifacts? I guess
your tool just injects FPIN messages with no performance impact,
resulting in this behaviour...)

But if you want to exclude marginal paths from queue depth:
by all means, go for it.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich

RE: [PATCH v8 8/8] nvme-multipath: queue-depth support for marginal paths

[Muneendra Kumar]

Correct me if iam wrong.
>>. In the case where 
>> all paths are marginal and no optimized or non-optimized path is 
>> found, we fall back to __nvme_find_path which selects the best marginal path

With the current patch __nvme_find_path will allways picks the path from non-optimized path ?

Regards,
Muneendra


>On 7/10/25 00:03, John Meneghini wrote:
>> Hannes, this patch fixes the queue-depth scheduler.  Please take a look.
>> 
>> On 7/9/25 5:26 PM, Bryan Gurney wrote:
>>> From: John Meneghini <jmeneghi@redhat.com>
>>>
>>> Exclude marginal paths from queue-depth io policy. In the case where 
>>> all paths are marginal and no optimized or non-optimized path is 
>>> found, we fall back to __nvme_find_path which selects the best marginal path.
>>>
>>> Tested-by: Bryan Gurney <bgurney@redhat.com>
>>> Signed-off-by: John Meneghini <jmeneghi@redhat.com>
>>> ---
>>>   drivers/nvme/host/multipath.c | 7 ++++++-
>>>   1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/ 
>>> multipath.c index 8d4e54bb4261..767583e8454b 100644
>>> --- a/drivers/nvme/host/multipath.c
>>> +++ b/drivers/nvme/host/multipath.c
>>> @@ -420,6 +420,9 @@ static struct nvme_ns 
>>> *nvme_queue_depth_path(struct nvme_ns_head *head)
>>>           if (nvme_path_is_disabled(ns))
>>>               continue;
>>> +        if (nvme_ctrl_is_marginal(ns->ctrl))
>>> +            continue;
>>> +
>>>           depth = atomic_read(&ns->ctrl->nr_active);
>>>           switch (ns->ana_state) {
>>> @@ -443,7 +446,9 @@ static struct nvme_ns 
>>> *nvme_queue_depth_path(struct nvme_ns_head *head)
>>>               return best_opt;
>>>       }
>>> -    return best_opt ? best_opt : best_nonopt;
>>> +    best_opt = (best_opt) ? best_opt : best_nonopt;
>>> +
>>> +    return best_opt ? best_opt : __nvme_find_path(head, 
>>> +numa_node_id());
>>>   }
>>>   static inline bool nvme_path_is_optimized(struct nvme_ns *ns)
>> 
>
>Hmm. Not convinced. I would expect a 'marginal' path to behave different
>(performance-wise) than unaffected paths. And the queue-depth scheduler should be able to handle paths with different performance characteristics just fine.
>(Is is possible that your results are test artifacts? I guess your tool just injects FPIN messages with no performance impact, resulting in this behaviour...)
>
>But if you want to exclude marginal paths from queue depth:
>by all means, go for it.
>



-- 
This electronic communication and the information and any files transmitted 
with it, or attached to it, are confidential and are intended solely for 
the use of the individual or entity to whom it is addressed and may contain 
information that is confidential, legally privileged, protected by privacy 
laws, or otherwise restricted from disclosure to anyone else. If you are 
not the intended recipient or the person responsible for delivering the 
e-mail to the intended recipient, you are hereby notified that any use, 
copying, distributing, dissemination, forwarding, printing, or copying of 
this e-mail is strictly prohibited. If you received this e-mail in error, 
please return the e-mail to the sender, delete it from your computer, and 
destroy any printed copy of it.

Re: [PATCH v8 8/8] nvme-multipath: queue-depth support for marginal paths

[John Meneghini]

On 7/10/25 10:59 PM, Muneendra Kumar wrote:
> Correct me if iam wrong.
>>> . In the case where
>>> all paths are marginal and no optimized or non-optimized path is
>>> found, we fall back to __nvme_find_path which selects the best marginal path
> 
> With the current patch __nvme_find_path will allways picks the path from non-optimized path ?

Not necessarily. I think it all comes down the this code:

                switch (ns->ana_state) {
                 case NVME_ANA_OPTIMIZED:
                         if (!nvme_ctrl_is_marginal(ns->ctrl)) {
                                 if (distance < found_distance) {
                                         found_distance = distance;
                                         found = ns;
                                 }
                                 break;
                         }
                         fallthrough;
                 case NVME_ANA_NONOPTIMIZED:
                         if (distance < fallback_distance) {
                                 fallback_distance = distance;
                                 fallback = ns;
                         }
                         break;

Any NVME_ANA_OPTIMIZED path that is marginal becomes a part of the fallback ns algorithm.

In the case where there is at least one NVME_ANA_OPTIMIZED path, it works correctly.  You will always find the NVME_ANA_OPTIMIZED
path.  In the case there there are no NVME_ANA_OPTIMIZED paths it turns in to kind of a crap shoot. You end up with the first fallback
ns that's found.  That could be an NVME_ANA_OPTIMIZED path or an NVME_ANA_NONOPTIMIZED path.  It all depends upon how the head->list is
sorted and if there are any disabled paths.

In our testing I've seen that this sometimes selects the NVME_ANA_OPTIMIZED path and sometimes the NVME_ANA_NONOPTIMIZED path.

In the simple test case, when the first two paths are optimized, and only one is marginal, this algorithm always selects the NVME_ANA_NONOPTIMIZED path.

It's only the more complicated test when all NVME_ANA_NONOPTIMIZED paths are marginal that I see some unpredictability.

/John