Re: [PATCH net-next 0/2] Add mqprio hardware offload support in hns3 driver

[Yunsheng Lin <linyunsheng@huawei.com>]

Hi, Yuval

On 2017/10/15 13:14, Yuval Mintz wrote:
>> Hi, Yuval
>>
>> On 2017/10/13 4:21, Yuval Mintz wrote:
>>>> This patchset adds a new hardware offload type in mqprio before adding
>>>> mqprio hardware offload support in hns3 driver.
>>>
>>> I think one of the biggest issues in tying this to DCB configuration is the
>>> non-immediate [and possibly non persistent] configuration.
>>>
>>> Scenario #1:
>>> User is configuring mqprio offloaded with 3 TCs while device is in willing
>> mode.
>>> Would you expect the driver to immediately respond with a success or
>> instead
>>> delay the return until the DCBx negotiation is complete and the operational
>>> num of TCs is actually 3?
>>
>> Well, when user requsts the mqprio offloaded by a hardware shared by DCB,
>> I expect
>> the user is not using the dcb tool.
>> If user is still using dcb tool, then result is undefined.
>>
>> The scenario you mention maybe can be enforced by setting willing to zero
>> when user
>> is requesting the mqprio offload, and restore the willing bit when unloaded
>> the mqprio
>> offload.
> 
> Sounds a bit harsh but would probably work.
> 
>> But I think the real issue is that dcb and mqprio shares the tc system in the
>> stack,
>> the problem may be better to be fixed in the stack rather than in the driver,
>> as you
>> suggested in the DCB patchset. What do you think?
> 
> What did you have in mind?

I was thinking maybe the tc system can provide a notification to mqprio and dcb.
mqprio and dcb register a callback to the tc system, when there is some change of
tc configuration, the tc system call the callback from mqprio and dcb.

> 
>>
>>>
>>> Scenario #2:
>>> Assume user explicitly offloaded mqprio with 3 TCs, but now DCB
>> configuration
>>> has changed on the peer side and 4 TCs is the new negotiated operational
>> value.
>>> Your current driver logic would change the number of TCs underneath the
>> user
>>> configuration [and it would actually probably work due to mqprio being a
>> crappy
>>> qdisc]. But was that the user actual intention?
>>> [I think the likely answer in this scenario is 'yes' since the alternative is no
>> better.
>>> But I still thought it was worth mentioning]
>>
>> You are right, the problem also have something to do with mqprio and dcb
>> sharing
>> the tc in the stack.
>>
>> Druing testing, when user explicitly offloaded mqprio with 3 TCs, all
>> queue has a default pfifo mqprio attached, after DCB changes the tc num to
>> 4,
>> using tc qdisc shows some queue does not have a default pfifo mqprio
>> attached.
> 
> Really? Then what did it show? 
> [I assume it has some pfifo attached, and it's an mqprio dump kind of an issue]

When queue size of the ndev is 16 and tc num is 3, we set the real queue size to
15 ( 5 * 3 = 15), mqprio only attach pfifo to the first 15 queue, when tc num change
to 4 by DCB, we set the real queue size to 16 (4 * 4 = 16).
So tc qdisc shows the last queue has no qdisc attached.

> 
>>
>> Maybe we can add a callback to notify mqprio the configuration has changed.
>>
> 
> Which would do what?
> You already have the notifications available for monitoring using dcbnl logic if the
> configuration change [for user]; So user can re-configure whatever it wants.

Yes, if user is only using dcb tool.

> But other than dropping all the qdisc configurations and going back to the default
> qdiscs, what default action would mqprio be able to do when configuration changes
> that actually makes sense?

As explained above, after dcb changing the configuration, some queue may have no qdisc
attached, so I was thinking maybe we can add pfifo to it if there is no qdsic attached
to it.

Thanks,
Yunsheng Lin

> 
>> Thanks
>> Yunsheng Lin
>>
>>>
>>> Cheers,
>>> Yuval
>>>
>>>>
>>>> Yunsheng Lin (2):
>>>>   mqprio: Add a new hardware offload type in mqprio
>>>>   net: hns3: Add mqprio hardware offload support in hns3 driver
>>>>
>>>>  drivers/net/ethernet/hisilicon/hns3/hnae3.h        |  1 +
>>>>  .../net/ethernet/hisilicon/hns3/hns3pf/hclge_dcb.c | 23 +++++++++++
>>>>  .../net/ethernet/hisilicon/hns3/hns3pf/hns3_enet.c | 46
>> ++++++++++++++-
>>>> -------
>>>>  include/uapi/linux/pkt_sched.h                     |  1 +
>>>>  4 files changed, 55 insertions(+), 16 deletions(-)
>>>>
>>>> --
>>>> 1.9.1
>>>
>>>
>>>
>