Re: TCPBacklogDrops during aggressive bursts of traffic

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2012-05-23 at 14:19 -0700, Alexander Duyck wrote:
> On 05/23/2012 10:10 AM, Alexander Duyck wrote:
> > On 05/23/2012 09:39 AM, Eric Dumazet wrote:
> >> On Wed, 2012-05-23 at 18:12 +0200, Eric Dumazet wrote:
> >>
> >>> With current driver, a MTU=1500 frame uses :
> >>>
> >>> sk_buff (256 bytes)
> >>> skb->head : 1024 bytes  (or more exaclty now : 512 + 384)
> >> By the way, NET_SKB_PAD adds 64 bytes so its 64 + 512 + 384 = 960
> > Actually pahole seems to be indicating to me the size of skb_shared_info
> > is 320, unless something has changed in the last few days.
> >
> > When I get a chance I will try to remember to reduce the ixgbe header
> > size to 256 which should also help.  The only reason it is set to 512
> > was to deal with the fact that the old alloc_skb code wasn't aligning
> > the shared info with the end of whatever size was allocated and so the
> > 512 was an approximation to make better use of the 1K slab allocation
> > back when we still were using hardware packet split.  That should help
> > to improve the page utilization for the headers since that would
> > increase the uses of a page from 4 to 6 for the skb head frag, and it
> > would drop truesize by another 256 bytes.
> >
> > Thanks,
> >
> > Alex
> Here is the patch for review.  I have submitted the official patch to Jeff 
> so that it can go through his tree for testing, validation, and submission 
> once Dave's tree opens back up.
> 
> ---
> 
> The recent changes to netdev_alloc_skb actually make it so that the size of
> the buffer now actually has a more direct input on the truesize.  So in
> order to make best use of the piece of a page we are allocated I am
> reducing the IXGBE_RX_HDR_SIZE to 256 so that our truesize will be reduced
> by 256 bytes as well.
> 
> This should result in performance improvements since the number of uses per
> page should increase from 4 to 6 in the case of a 4K page.  In addition we
> should see socket performance improvements due to the truesize dropping
> to less than 1K for buffers less than 256 bytes.
> 
> Not-Yet-Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
> ---
> 
>  drivers/net/ethernet/intel/ixgbe/ixgbe.h      |   15 ++++++++-------
>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c |    4 ++--
>  2 files changed, 10 insertions(+), 9 deletions(-)
> 
> 
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe.h b/drivers/net/ethernet/intel/ixgbe/ixgbe.h
> index 402dd66..468e4ab 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe.h
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe.h
> @@ -77,17 +77,18 @@
>  #define IXGBE_MAX_FCPAUSE		 0xFFFF
>  
>  /* Supported Rx Buffer Sizes */
> -#define IXGBE_RXBUFFER_512   512    /* Used for packet split */
> +#define IXGBE_RXBUFFER_256    256  /* Used for skb receive header */
>  #define IXGBE_MAX_RXBUFFER  16384  /* largest size for a single descriptor */
>  
>  /*
> - * NOTE: netdev_alloc_skb reserves up to 64 bytes, NET_IP_ALIGN mans we
> - * reserve 2 more, and skb_shared_info adds an additional 384 bytes more,
> - * this adds up to 512 bytes of extra data meaning the smallest allocation
> - * we could have is 1K.
> - * i.e. RXBUFFER_512 --> size-1024 slab
> + * NOTE: netdev_alloc_skb reserves up to 64 bytes, NET_IP_ALIGN means we
> + * reserve 64 more, and skb_shared_info adds an additional 320 bytes more,
> + * this adds up to 448 bytes of extra data.
> + *
> + * Since netdev_alloc_skb now allocates a page fragment we can use a value
> + * of 256 and the resultant skb will have a truesize of 960 or less.
>   */
> -#define IXGBE_RX_HDR_SIZE IXGBE_RXBUFFER_512
> +#define IXGBE_RX_HDR_SIZE IXGBE_RXBUFFER_256
>  
>  #define MAXIMUM_ETHERNET_VLAN_SIZE (ETH_FRAME_LEN + ETH_FCS_LEN + VLAN_HLEN)
>  
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> index 7f92e40..f92b31a 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> @@ -1520,8 +1520,8 @@ static bool ixgbe_cleanup_headers(struct ixgbe_ring *rx_ring,
>  	 * 60 bytes if the skb->len is less than 60 for skb_pad.
>  	 */
>  	pull_len = skb_frag_size(frag);
> -	if (pull_len > 256)
> -		pull_len = ixgbe_get_headlen(va, pull_len);
> +	if (pull_len > IXGBE_RX_HDR_SIZE)
> +		pull_len = ixgbe_get_headlen(va, IXGBE_RX_HDR_SIZE);
>  
>  	/* align pull length to size of long to optimize memcpy performance */
>  	skb_copy_to_linear_data(skb, va, ALIGN(pull_len, sizeof(long)));
> 


By the way you should reword the comment about NET_IP_ALIGN

On x86 NET_IP_ALIGN is 0, so we dont 'reserve 64 bytes more'


-> 896 bytes

Also, are you sure :

srrctl |= (IXGBE_RX_HDR_SIZE << IXGBE_SRRCTL_BSIZEHDRSIZE_SHIFT) &  
	IXGBE_SRRCTL_BSIZEHDR_MASK; 


is still needed in ixgbe_configure_srrctl() , since it uses
IXGBE_SRRCTL_DESCTYPE_ADV_ONEBUF (non packet split)