From: Pavel Machek <pa...@ucw.cz> Date: Thu, 24 Nov 2016 09:55:06 +0100
> Hi! > >> 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.
