netdev - Re: [PATCH 3/3]: tg3: Manage TX backlog in-driver.

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Date:	Sat, 21 Jun 2008 10:02:03 -0700
From:	"Michael Chan" <mchan@...adcom.com>
To:	"'David Miller'" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
cc:	"vinay@...ux.vnet.ibm.com" <vinay@...ux.vnet.ibm.com>,
	"krkumar2@...ibm.com" <krkumar2@...ibm.com>
Subject: Re: [PATCH 3/3]: tg3: Manage TX backlog in-driver.

David Miller wrote:

> +       /* This is a deadlock breaker.  tg3_tx() updates the consumer
> +        * index, then checks the tx_backlog for emptiness.  It also
> +        * tries to mitigate work by only flushing the backlog when at
> +        * least a certain percentage of space is available.  Those
> +        * tests in tg3_tx() run lockless.
> +        *
> +        * Here, we make the two primary memory operations in the
> +        * reverse order.  The idea is to make sure that one of these
> +        * two code paths will process the backlog no matter what the
> +        * order of their relative execution might be.
> +        *
> +        * In short:
> +        *
> +        * tg3_tx() --> tp->tx_cons = foo; test skb_queue_empty()
> +        * tg3_start_xmit() --> __skb_queue_tail(); test tp->tx_cons
> +        */
> +       if (tg3_tx_avail(tp) > TG3_TX_WAKEUP_THRESH(tp))
> +               __tg3_xmit_backlog(tp);
> +}
>

I'm thinking that we should use a different wakeup threshold here
than the one in tg3_tx().  If the 2 threads test for the same
threshold, it increases the likelihood that they will find the
condition to be true at the same time.  This contention will
cause tg3_tx() to spin longer for the tx_lock, because
tg3_start_xmit() will call __tg3_xmit_backlog() and keep the
tx_lock longer.

Since 99% of the wakeup should be done in tg3_tx(), we should
increase the threshold here to perhaps 3/4 of the ring.

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