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

From: Pavel Machek
Date: Thu Nov 24 2016 - 16:25:47 EST


> >> 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, ...
> > 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.

Ok, I'd disable coalescing, but could not figure it out till. What is
generic way to do that?

It seems only thing stmmac_tx_timer() does is calling
stmmac_tx_clean(), which reclaims tx_skbuff[] entries. It should be
possible to do that explicitely, without delay, but it stops working
completely if I attempt to do that.

On a side note, stmmac_poll() does stmmac_enable_dma_irq() while
stmmac_dma_interrupt() disables interrupts. But I don't see any
protection between the two, so IMO it could race and we'd end up
without polling or interrupts...

Thanks and best regards,
(cesky, pictures)

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