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

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Souradeep Chakrabarti <schakrabarti@microsoft.com>
Date: Thu, 6 Jul 2023 11:43:58 +0000

> 
> 
>> -----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;
>> cai.huoqing@linux.dev; ssengar@linux.microsoft.com; vkuznets@redhat.com;
>> tglx@linutronix.de; linux-hyperv@vger.kernel.org; netdev@vger.kernel.org;
>> linux-kernel@vger.kernel.org; linux-rdma@vger.kernel.org;
>> stable@vger.kernel.org
>> 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.

This waiting loop is needed to let the pending Tx packets be sent. If
they weren't sent in 1 second, it most likely makes no sense already
whether they will be sent at all or not -- the destination host won't
wait for them for so long.
You say that it may happen only in case of HW issue. If so, I assume you
need to fix it some way, e.g. do a HW reset or so? If so, why bother
waiting for Tx completions if Tx is hung? You free all skbs later either
way, so there are no leaks.

>>
>>>>> 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

Thanks,
Olek