Re: [PATCH v3 18/21] nvme: Update CCR completion wait timeout to consider CQT

[James Smart <jsmart833426@gmail.com>]

On 2/17/2026 7:35 AM, Mohamed Khalfella wrote:
> On Tue 2026-02-17 08:09:33 +0100, Hannes Reinecke wrote:
>> On 2/16/26 19:45, Mohamed Khalfella wrote:
>>> On Mon 2026-02-16 13:54:18 +0100, Hannes Reinecke wrote:
>>>> On 2/14/26 05:25, Mohamed Khalfella wrote:
>>>>> TP8028 Rapid Path Failure Recovery does not define how much time the
>>>>> host should wait for CCR operation to complete. It is reasonable to
>>>>> assume that CCR operation can take up to ctrl->cqt. Update wait time for
>>>>> CCR operation to be max(ctrl->cqt, ctrl->kato).
>>>>>
>>>>> Signed-off-by: Mohamed Khalfella <mkhalfella@purestorage.com>
>>>>> ---
>>>>>     drivers/nvme/host/core.c | 2 +-
>>>>>     1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
>>>>> index 0680d05900c1..ff479c0263ab 100644
>>>>> --- a/drivers/nvme/host/core.c
>>>>> +++ b/drivers/nvme/host/core.c
>>>>> @@ -631,7 +631,7 @@ static int nvme_issue_wait_ccr(struct nvme_ctrl *sctrl, struct nvme_ctrl *ictrl)
>>>>>     	if (result & 0x01) /* Immediate Reset Successful */
>>>>>     		goto out;
>>>>>     
>>>>> -	tmo = secs_to_jiffies(ictrl->kato);
>>>>> +	tmo = msecs_to_jiffies(max(ictrl->cqt, ictrl->kato * 1000));
>>>>>     	if (!wait_for_completion_timeout(&ccr.complete, tmo)) {
>>>>>     		ret = -ETIMEDOUT;
>>>>>     		goto out;
>>>>
>>>> That is not my understanding. I was under the impression that CQT is the
>>>> _additional_ time a controller requires to clear out outstanding
>>>> commands once it detected a loss of communication (ie _after_ KATO).
>>>> Which would mean we have to wait for up to
>>>> (ctrl->kato * 1000) + ctrl->cqt.
>>>
>>> At this point the source controller knows about communication loss. We
>>> do not need kato wait. In theory we should just wait for CQT.
>>> max(cqt, kato) is a conservative guess I made.
>>>
>> Not quite. The source controller (on the host!) knows about the
>> communication loss. But the target might not, as the keep-alive
>> command might have arrived at the target _just_ before KATO
>> triggered on the host. So the target is still good, and will
>> be waiting for _another_ KATO interval before declaring
>> a loss of communication.
>> And only then will the CQT period start at the target.
>>
>> Randy, please correct me if I'm wrong ...
>>
> 
> wait_for_completion_timeout(&ccr.complete, tmo)) waits for CCR operation
> to complete. The wait starts after CCR command completed successfully.
> IOW, it starts after the host received a CQE from source controller on
> the target telling us all is good. If the source controller on the target
> already know about loss of communication then there is no need to wait
> for KATO. We just need to wait for CCR operation to finish because we
> know it has been started successfully.
> 
> The specs does not tell us how much time to wait for CCR operation to
> complete. max(cqt, kato) is an estimate I think reasonable to make.

So, we've sent CCR, received a CQE for the CCR within KATO (timeout in 
nvme_issue_wait_ccr()), then are waiting another max(KATO, CQT) for the 
io to die.

As CQT is the time to wait once the ctrl is killing the io, and as the 
response indicated it certainly passed that point, a minimum of CQT 
should be all that is needed. Why are we bringing KATO into the picture?

-- this takes me over to patch 8 and the timeout on CCR response being KATO:
Why is KATO being used ? nothing about getting the response says it is 
related to the keep alive. Keepalive can move along happily while CCR 
hangs out and really has nothing to do with KATO.

If using the rationale of a keepalive cmd processing - has roundtrip 
time and minimal and prioritized processing, as CCR needs to do more and 
as the spec allows holding on to always return 1, it should be 
KATO+<something>, where <something> is no more than CQT.

But given that KATO can be really long as its trying to catch 
communication failures, and as our ccr controller should not have comm 
issues, it should be fairly quick. So rather than a 2min KATO, why not 
10-15s ? This gets a little crazy as it takes me down paths of why not 
fire off multiple CCRs (via different ctlrs) to the subsystem at short 
intervals (the timeout) to finally find one that completes quickly and 
then start CQT. And if nothing completes quickly bound the whole thing 
to fencing start+KATO+CQT ?

-- james