-----Original Message-----
From: Alexander Lobakin <aleksander.lobakin@intel.com>
Sent: Thursday, July 6, 2023 7:49 AM
To: Souradeep Chakrabarti <redacted>; souradeep
chakrabarti [off-list ref]
Cc: KY Srinivasan <kys@microsoft.com>; Haiyang Zhang
[off-list ref]; wei.liu@kernel.org; Dexuan Cui
[off-list ref]; davem@davemloft.net; edumazet@google.com;
kuba@kernel.org; pabeni@redhat.com; Long Li [off-list ref]; Ajay
Sharma [off-list ref]; 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 <redacted>
Date: Thu, 6 Jul 2023 11:43:58 +0000

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-----Original Message-----
From: Alexander Lobakin <aleksander.lobakin@intel.com>
Sent: Thursday, July 6, 2023 5:09 PM
To: Souradeep Chakrabarti <redacted>; souradeep
chakrabarti [off-list ref]
Cc: KY Srinivasan <kys@microsoft.com>; Haiyang Zhang
[off-list ref]; wei.liu@kernel.org; Dexuan Cui
[off-list ref]; davem@davemloft.net; edumazet@google.com;
kuba@kernel.org; pabeni@redhat.com; Long Li [off-list ref];

Ajay

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Sharma [off-list ref]; leon@kernel.org;
cai.huoqing@linux.dev; ssengar@linux.microsoft.com;

vkuznets@redhat.com;

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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 <redacted>
Date: Thu, 6 Jul 2023 10:41:03 +0000

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-----Original Message-----
From: Alexander Lobakin <aleksander.lobakin@intel.com>
Sent: Wednesday, July 5, 2023 8:06 PM

[...]

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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.

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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.

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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

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it :

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2346     /* No packet can be transmitted now since apc->port_is_up is

false.

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2347      * There is still a tiny chance that mana_poll_tx_cq() can re-

enable

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2348      * a txq because it may not timely see apc->port_is_up being

cleared

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2349      * to false, but it doesn't matter since mana_start_xmit() drops

any

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2350      * new packets due to apc->port_is_up being false.

The value 120 seconds has been decided here based on maximum

number

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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.

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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.

At that point, we don't actually care if the pending packets are sent or not. 
But if we free the queues too soon, and the HW is slow for unexpected 
reasons, a delayed completion notice will DMA into the freed memory and 
cause corruption. That's why we have a longer waiting time.

Souradeep, you may double check with hostnet team to see what's the 
best waiting time to ensure no more HW activities.

Thanks,
- Haiyang