RE: [EXTERNAL] Re: [PATCH V4 net] net: mana: Fix MANA VF unload when host is unresponsive

[Souradeep Chakrabarti <schakrabarti@microsoft.com>]

>-----Original Message-----
>From: Alexander Lobakin <aleksander.lobakin@intel.com>
>Sent: Thursday, July 6, 2023 5:09 PM
>To: Souradeep Chakrabarti <schakrabarti@microsoft.com>; souradeep
>chakrabarti <schakrabarti@linux.microsoft.com>
>Cc: KY Srinivasan <kys@microsoft.com>; Haiyang Zhang
><haiyangz@microsoft.com>; wei.liu@kernel.org; Dexuan Cui
><decui@microsoft.com>; davem@davemloft.net; edumazet@google.com;
>kuba@kernel.org; pabeni@redhat.com; Long Li <longli@microsoft.com>; Ajay
>Sharma <sharmaajay@microsoft.com>; leon@kernel.org;
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>Subject: Re: [EXTERNAL] Re: [PATCH V4 net] net: mana: Fix MANA VF unload
>when host is unresponsive
>
>From: Souradeep Chakrabarti <schakrabarti@microsoft.com>
>Date: Thu, 6 Jul 2023 10:41:03 +0000
>
>>
>>
>>> -----Original Message-----
>>> From: Alexander Lobakin <aleksander.lobakin@intel.com>
>>> Sent: Wednesday, July 5, 2023 8:06 PM
>
>[...]
>
>>>>> 120 seconds by 2 msec step is 60000 iterations, by 1 msec is 120000
>>>>> iterations. I know usleep_range() often is much less precise, but still.
>>>>> Do you really need that much time? Has this been measured during
>>>>> the tests that it can take up to 120 seconds or is it just some
>>>>> random value that "should be enough"?
>>>>> If you really need 120 seconds, I'd suggest using a timer / delayed
>>>>> work instead of wasting resources.
>>>> Here the intent is not waiting for 120 seconds, rather than avoid
>>>> continue checking the pending_sends  of each tx queues for an
>>>> indefinite time,
>>> before freeing sk_buffs.
>>>> The pending_sends can only get decreased for a tx queue,  if
>>>> mana_poll_tx_cq() gets called for a completion notification and
>>>> increased by
>>> xmit.
>>>>
>>>> In this particular bug, apc->port_is_up is not set to false, causing
>>>> xmit to keep increasing the pending_sends for the queue and
>>>> mana_poll_tx_cq() not getting called for the queue.
>>>>
>>>> If we see the comment in the function mana_dealloc_queues(), it mentions
>it :
>>>>
>>>> 2346     /* No packet can be transmitted now since apc->port_is_up is false.
>>>> 2347      * There is still a tiny chance that mana_poll_tx_cq() can re-enable
>>>> 2348      * a txq because it may not timely see apc->port_is_up being cleared
>>>> 2349      * to false, but it doesn't matter since mana_start_xmit() drops any
>>>> 2350      * new packets due to apc->port_is_up being false.
>>>>
>>>> The value 120 seconds has been decided here based on maximum number
>>>> of queues
>>>
>>> This is quite opposite to what you're saying above. How should I
>>> connect these
>>> two:
>>>
>>> Here the intent is not waiting for 120 seconds
>>>
>>> +
>>>
>>> The value 120 seconds has been decided here based on maximum number
>>> of queues
>>>
>>> ?
>>> Can cleaning the Tx queues really last for 120 seconds?
>>> My understanding is that timeouts need to be sensible and not go to
>>> the outer space. What is the medium value you got during the tests?
>>>
>> For each queue each takes few milli second, in a normal condition. So
>> based on maximum number of allowed queues for our h/w it won't go
>> beyond a sec.
>> The 120s only happens rarely during some NIC HW issue -unexpected.
>> So this timeout will only trigger in a very rare scenario.
>
>So set the timeout to 2 seconds if it makes no difference?
It can go near 120 seconds in a very rare MANA h/w scenario. That normally won't happen.
But during that scenario, we may need 120 seconds.
>
>>>> are allowed in this specific hardware, it is a safe assumption.
>>>>>
>>>>>>
>>>>>> +   for (i = 0; i < apc->num_queues; i++) {
>>>>>> +           txq = &apc->tx_qp[i].txq;
>>>>>> +           cq = &apc->tx_qp[i].tx_cq;
>[...]
>
>Thanks,
>Olek