Re: stmmac ethernet in kernel 4.4: coalescing related pauses?

From: Lino Sanfilippo
Date: Mon Nov 28 2016 - 08:09:18 EST

Hi Pavel,

On 23.11.2016 11:51, Pavel Machek wrote:

I'm debugging strange delays during transmit in stmmac driver. They
seem to be present in 4.4 kernel (and older kernels, too). Workload is
burst of udp packets being sent, pause, burst of udp packets, ...

Test code is attached, I use these parameters for testing:

./udp-test raw 1234 1000 100 30

The delays seem to be related to coalescing:

#define STMMAC_COAL_TX_TIMER 40000
#define STMMAC_MAX_COAL_TX_TICK 100000

If I lower the parameters, delays are gone, but I get netdev watchdog
backtrace followed by broken driver.

Any ideas what is going on there?

[I'm currently trying to get newer kernels working on affected

Best regards,


I once encountered a similar behaviour with a driver. The reason was that the socket
queue limit was temporarily exhausted because the irq handler did not free the tx skbs
fast enough (that driver also used irq coalescing).
Calling skb_orphan() in the xmit handler made this issue disappear.