netdev - Re: b44: Reset due to FIFO overflow.

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Date:	Mon, 28 Jun 2010 14:21:18 -0700
From:	Mitchell Erblich <erblichs@...thlink.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	James Courtier-Dutton <james.dutton@...il.com>,
	netdev@...r.kernel.org
Subject: Re: b44: Reset due to FIFO overflow.


On Jun 28, 2010, at 4:09 AM, Eric Dumazet wrote:

> Le lundi 28 juin 2010 à 11:17 +0100, James Courtier-Dutton a écrit :
>> On 28 June 2010 11:00, Eric Dumazet <eric.dumazet@...il.com> wrote:
>>> 
>>> Problem is we receive a spike of RX network frames (possibly UDP or some
>>> other RX only trafic), and chip raises an RX fifo overflow _error_
>>> indication.
>>> 

IMO, spikes are a normal behaviour.

>> 
>> The cause of the RX overflow is in my case is TCP.
>> It is reproducible in mythtv.
>> While watching LiveTV, press "s" for the program guide.
>> The program guide is implemented into mythtv by a SQL query that
>> results in a large response.
>> The kernel is probably not servicing the RX FIFO quickly enough due to
>> it being busy doing something else. In this case, probably a video
>> mode switch.
>> 
> 
> Thats strange, b44 has a big RX ring... and tcp sender should wait for
> ACK...
> 

Slow start, etc SHOULD/CAN  double the number of in-flight segments in each
next round-trip, placing them back to back.

IMO,  a stress test, would be a large number/wirespeed set of pings?

>>> Some hardware are buggy enough that such error indication is fatal and
>>> _require_ hardware reset. Thats life. I suspect b44 driver doing a full
>>> reset is not a random guess from driver author, but to avoid a complete
>>> NIC lockup.
>>> 
>> 
>> Interesting, which hardware, apart from the b44, is it that "requires"
>> a hardware reset after a RX FIFO overflow.
> 
> Just take a look at some net drivers and you'll see some of them have
> this requirement.
> 
> rtl8169_rx_interrupt()
> ...
> 	if (status & RxFOVF) {
> 		rtl8169_schedule_work(dev, rtl8169_reset_task);
> 		dev->stats.rx_fifo_errors++;
> 	}
> 
> 
> 
> 


If they can reset in say X frame loss units, then why not reset if
X is an acceptable number?

And a hammer may fix the dent, while I may be more
interested in preventing the dent in the first place.

Mitchell Erblich





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