netdev - Re: [PATCH RFC 5/5] net:stmmac: asynchronous tx

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Date:	Mon, 21 Oct 2013 09:30:05 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Giuseppe CAVALLARO <peppe.cavallaro@...com>
Cc:	Jimmy Perchet <jimmy.perchet@...rot.com>, netdev@...r.kernel.org
Subject: Re: [PATCH RFC 5/5] net:stmmac: asynchronous tx_clean

On Mon, 2013-10-21 at 15:52 +0200, Giuseppe CAVALLARO wrote:
> Hello Jimmy
> 
> On 10/16/2013 5:24 PM, Jimmy Perchet wrote:
> > Tx descriptor's cleanup and preparation are serialized, which is not necessary
> > and decrease performance.
> > In addition TX descriptor's cleanup is performed on NET_RX softirq, this is
> > confusing.
> 
> hmm, here you are changing the logic behind the tx/rx processes.
> 
> As done in many drivers, the stmmac cleans the tx resources in
> NAPI context and this is not a confuse approach ;-).
> 
> It gave me some performance improvements especially on TCP benchmarks.
> 
> > This patch unserialize tx descriptor's cleanup and preparation
> > and defer cleanup in workqueue.
> 
> So you decide to use workqueue and I kindly ask you to give me more
> details about the performance improvements (UDP/TCP) and cpu usage.
> 
> I can try to do some tests on my side too. This could take a while
> unfortunately.

Anyway this patch is buggy.

1) Removing tx_lock spinlock in TX completion adds a race in
stmmac_xmit()

2) Generally speaking, we should not rely on a work queue to perform TX
completions.

Think about being flooded by incoming frames.

Work queue could never be scheduled.


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