Subject: Re: TCPBacklogDrops during aggressive bursts of traffic

From: Alexander Duyck <[email-masked]>
Date: Wed, 23 May 2012 14:19:38 -0700

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 <[email-masked]>
---

 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)));