Re: stmmac ethernet in kernel 4.9-rc6: coalescing related pauses.

From: Pavel Machek <redacted>
Date: Thu, 24 Nov 2016 09:55:06 +0100

Hi!

quoted

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 10.0.0.6 1234 1000 100 30

The delays seem to be related to coalescing:

drivers/net/ethernet/stmicro/stmmac/common.h
#define STMMAC_COAL_TX_TIMER    40000
#define STMMAC_MAX_COAL_TX_TICK 100000
#define STMMAC_TX_MAX_FRAMES    256

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?

4.9-rc6 still has the delays. With the

#define STMMAC_COAL_TX_TIMER 1000
#define STMMAC_TX_MAX_FRAMES 2

settings, delays go away, and driver still works. (It fails fairly
fast in 4.4). Good news. But the question still is: what is going on
there?

256 packets looks way too large for being a trigger for aborting the
TX coalescing timer.

Looking more deeply into this, the driver is using non-highres timers
to implement the TX coalescing.  This simply cannot work.

1 HZ, which is the lowest granularity of non-highres timers in the
kernel, is variable as well as already too large of a delay for
effective TX coalescing.

I seriously think that the TX coalescing support should be ripped out
or disabled entirely until it is implemented properly in this driver.