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

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Message-ID: <20161124085506.GA25007@amd>
Date:   Thu, 24 Nov 2016 09:55:06 +0100
From:   Pavel Machek <pavel@....cz>
To:     peppe.cavallaro@...com, netdev@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>
Subject: stmmac ethernet in kernel 4.9-rc6: coalescing related pauses.

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?

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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