[LSF/MM/BPF BOF] Userspace command abouts

[Damien Le Moal]

On 2/28/23 01:33, Sagi Grimberg wrote:
> 
>>> On Fri, Feb 24, 2023 at 11:54:39PM +0000, Chaitanya Kulkarni wrote:
>>>> I do think that we should work on CDL for NVMe as it will solve some of
>>>> the timeout related problems effectively than using aborts or any other
>>>> mechanism.
>>>
>>> That proposal exists in NVMe TWG, but doesn't appear to have recent activity.
>>> The last I heard, one point of contention was where the duration limit property
>>> exists: within the command, or the queue. From my perspective, if it's not at
>>> the queue level, the limit becomes meaningless, but hey, it's not up to me.
>>
>> Limit attached to the command makes things more flexible and easier for the
>> host, so personally, I prefer that. But this has an impact on the controller:
>> the device needs to pull in *all* commands to be able to know the limits and do
>> scheduling/aborts appropriately. That is not something that the device designers
>> like, for obvious reasons (device internal resources...).
>>
>> On the other hand, limits attached to queues could lead to either a serious
>> increase in the number of queues (PCI space & number of IRQ vectors limits), or,
>> loss of performance as a particular queue with the desired limit would be
>> accessed from multiple CPUs on the host (lock contention). Tricky problem I
>> think with lots of compromises.
> 
> I'm not up to speed on how CDL is defined, but I'm unclear how CDL at
> the queue level would cause the host to open more queues?

There would be the need for one queue pair per limit defined, in addition to the
regular "no limits" queue pairs. And given that CDL allows defining up to 7
limits for read AND write commands, if kept as-is, this means potentially 14
additional queue pairs shared among all CPUs, or even more than that if one
wants per CPU queues with limits.

> Another question, does CDL have any relationship with NVMe "Time Limited
> Error Recovery"? where the host can set a feature for timeout and
> indicate if the controller should respect it per command?

This NVMe feature does map to one of the possible limits that can be defined
with CDL. CDL currently allows 3 different limits:
 - Active time limit: limit on command execution involving media access
 - inactive time limit: limit on device internal queueing time before processing
of the command starts (aging control)
 - Duration guideline: overall limit on the command processing by the device

> While this is not a full-blown every queue/command has its own timeout,
> it could address the original use-case given by Hannes. And it's already
> there.

The above limits are what is currently defined in T10/T13 for SCSI/ATA devices.
NVMe may need some tweaks to get a better mapping to the different use cases.

-- 
Damien Le Moal
Western Digital Research