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: Wednesday, July 5, 2023 8:06 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;
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>Subject: Re: [EXTERNAL] Re: [PATCH V4 net] net: mana: Fix MANA VF unload
>when host is unresponsive
>
>[You don't often get email from aleksander.lobakin@intel.com. Learn why this is
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>
>From: Souradeep Chakrabarti <schakrabarti@microsoft.com>
>Date: Mon, 3 Jul 2023 19:55:06 +0000
>
>>
>>
>>> -----Original Message-----
>>> From: Alexander Lobakin <aleksander.lobakin@intel.com>
>>> Sent: Monday, July 3, 2023 10:18 PM
>
>[...]
>
>>>>     for (i = 0; i < apc->num_queues; i++) {
>>>>             txq = &apc->tx_qp[i].txq;
>>>> -
>>>> -           while (atomic_read(&txq->pending_sends) > 0)
>>>> +           while (atomic_read(&txq->pending_sends) > 0 &&
>>>> +                  time_before(jiffies, timeout)) {
>>>>                     usleep_range(1000, 2000);> +            }
>>>>     }
>>>
>>> 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.
>> 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;
>>>
>>> cq can be just &txq->tx_cq.
>> mana_txq  structure does not have a pointer to mana_cq.
>
>Sorry, misread, my bad.
>
>>>
>>>> +           while (atomic_read(&txq->pending_sends)) {
>>>> +                   skb = skb_dequeue(&txq->pending_skbs);
>>>> +                   mana_unmap_skb(skb, apc);
>>>> +                   napi_consume_skb(skb, cq->budget);
>>>
>>> (you already have comment about this one)
>>>
>>>> +                   atomic_sub(1, &txq->pending_sends);
>>>> +           }
>>>> +   }
>>>>     /* We're 100% sure the queues can no longer be woken up, because
>>>>      * we're sure now mana_poll_tx_cq() can't be running.
>>>>      */
>>>
>>> Thanks,
>>> Olek
>Thanks,
>Olek