netdev - Re: TCP transmit performance regression

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Message-ID: <CACVXFVNYwNrdWS_EiQ4O0-ou369AQ2tB4qbvmzY1rt5T3QnyUw@mail.gmail.com>
Date:	Mon, 9 Jul 2012 21:23:18 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Network Development <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: TCP transmit performance regression

On Thu, Jul 5, 2012 at 5:49 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2012-07-05 at 16:42 +0800, Ming Lei wrote:
>> On Thu, Jul 5, 2012 at 4:33 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Thu, 2012-07-05 at 16:27 +0800, Ming Lei wrote:
>> >
>> >> After some investigation, the problem is caused by enabling
>> >> DEBUG_SLAB, so it is not a regression.
>> >>
>> >
>> > Strange, unless your machine is a _very_ slow one maybe ?
>>
>> It is a beagle-xm board, and its cpu is ARMv7, 1GHz.
>
> OK, driver seems buggy, please try following patch (on both sides if
> possible)
>
>  drivers/net/usb/smsc95xx.c |   11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
> index b1112e7..0a4ae35 100644
> --- a/drivers/net/usb/smsc95xx.c
> +++ b/drivers/net/usb/smsc95xx.c
> @@ -1084,26 +1084,23 @@ static int smsc95xx_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
>                         if (skb->len == size) {
>                                 if (dev->net->features & NETIF_F_RXCSUM)
>                                         smsc95xx_rx_csum_offload(skb);
> -                               skb_trim(skb, skb->len - 4); /* remove fcs */
> +                               __skb_trim(skb, skb->len - 4); /* remove fcs */
>                                 skb->truesize = size + sizeof(struct sk_buff);
>
>                                 return 1;
>                         }
>
> -                       ax_skb = skb_clone(skb, GFP_ATOMIC);
> +                       ax_skb = netdev_alloc_skb_ip_align(dev->net, size);
>                         if (unlikely(!ax_skb)) {
>                                 netdev_warn(dev->net, "Error allocating skb\n");
>                                 return 0;
>                         }
>
> -                       ax_skb->len = size;
> -                       ax_skb->data = packet;
> -                       skb_set_tail_pointer(ax_skb, size);
> +                       memcpy(skb_put(ax_skb, size), packet, size);
>
>                         if (dev->net->features & NETIF_F_RXCSUM)
>                                 smsc95xx_rx_csum_offload(ax_skb);
> -                       skb_trim(ax_skb, ax_skb->len - 4); /* remove fcs */
> -                       ax_skb->truesize = size + sizeof(struct sk_buff);
> +                       __skb_trim(ax_skb, ax_skb->len - 4); /* remove fcs */
>
>                         usbnet_skb_return(dev, ax_skb);
>                 }
>
>

Looks the patch replaces skb_clone with netdev_alloc_skb_ip_align and
introduces extra copies on incoming data, so would you mind explaining
it in a bit detail? And why is skb_clone not OK for the purpose?


Thanks,
-- 
Ming Lei
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