netdev - Re: [PATCH net-next 3/4] bnx2: Remove some unnecessary smp

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Message-Id: <20100719.203119.106787548.davem@davemloft.net>
Date:	Mon, 19 Jul 2010 20:31:19 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mchan@...adcom.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-next 3/4] bnx2: Remove some unnecessary smp_mb() in
 tx fast path.

From: "Michael Chan" <mchan@...adcom.com>
Date: Mon, 19 Jul 2010 17:15:04 -0700

> smp_mb() inside bnx2_tx_avail() is used twice in the normal
> bnx2_start_xmit() path (see illustration below).  The full memory
> barrier is only necessary during race conditions with tx completion.
> We can speed up the tx path by replacing smp_mb() in bnx2_tx_avail()
> with a compiler barrier.  The compiler barrier is to force the
> compiler to fetch the tx_prod and tx_cons from memory.
> 
> In the race condition between bnx2_start_xmit() and bnx2_tx_int(),
> we have the following situation:
> 
> bnx2_start_xmit()                       bnx2_tx_int()
>     if (!bnx2_tx_avail())
>             BUG();
> 
>     ...
> 
>     if (!bnx2_tx_avail())
>             netif_tx_stop_queue();          update_tx_index();
>             smp_mb();                       smp_mb();
>             if (bnx2_tx_avail())            if (netif_tx_queue_stopped() &&
>                     netif_tx_wake_queue();      bnx2_tx_avail())
> 
> With smp_mb() removed from bnx2_tx_avail(), we need to add smp_mb() to
> bnx2_start_xmit() as shown above to properly order netif_tx_stop_queue()
> and bnx2_tx_avail() to check the ring index.  If it is not strictly
> ordered, the tx queue can be stopped forever.
> 
> This improves performance by about 5% with 2 ports running bi-directional
> 64-byte packets.
> 
> Reviewed-by: Benjamin Li <benli@...adcom.com>
> Reviewed-by: Matt Carlson <mcarlson@...adcom.com>
> Signed-off-by: Michael Chan <mchan@...adcom.com>

Applied.
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