[Cerowrt-devel] [PATCH] skb_reduce_truesize: helper function for shrinking skbs whenever needed

[Cerowrt-devel] [PATCH] skb_reduce_truesize: helper function for shrinking skbs whenever needed

[Dave Täht]

From: Dave Taht <dave.taht@bufferbloat.net>

On embedded devices in particular, large queues of small packets from the rx
path with a large truesize can exist. Reducing their size can reduce
memory pressure. skb_reduce_truesize is a helper function for doing this,
when needed.
---
 include/linux/skbuff.h |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 6a2c34e..2080cb1 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1671,6 +1671,24 @@ static inline void pskb_trim_unique(struct sk_buff *skb, unsigned int len)
 	BUG_ON(err);
 }
 
+/*
+ * Caller wants to reduce memory needs before queueing skb
+ * The (expensive) copy should not be be done in fast path.
+ */
+static inline struct sk_buff *skb_reduce_truesize(struct sk_buff *skb)
+{
+	if (skb->truesize > 2 * SKB_TRUESIZE(skb->len)) {
+		struct sk_buff *nskb;
+		nskb = skb_copy_expand(skb, skb_headroom(skb), 0,
+			GFP_ATOMIC | __GFP_NOWARN);
+		if (nskb) {
+			__kfree_skb(skb);
+			skb = nskb;
+		}
+	}
+	return skb;
+}
+
 /**
  *	skb_orphan - orphan a buffer
  *	@skb: buffer to orphan
-- 
1.7.9.5

Re: [Cerowrt-devel] [PATCH] skb_reduce_truesize: helper function for shrinking skbs whenever needed

[Dave Taht]

I have been carrying this patch out of tree for a while, this
refreshes it for linux 3.7.1.

I have subsequent patches that add this facility to *fq_codel,
pfifo_fast, and elsewhere.

It seemed to help somewhat when flooding the box, however I see all
sorts of interesting packet fragmentation/defragmentation/splitting
stuff happening throughout the 3.6 and 3.7 series of network stack and
I have no idea if it's as useful as all that, so I'm willing to drop
it and see what happens...

What I'd like better though is a multi-exit form of fq_codel that only
used one queue allocation for the 4 (or 5 if you count multicast)
queues in wifi... but next up is to port forward the prototype to
cake...

-- 
Dave Täht

Fixing bufferbloat with cerowrt: http://www.teklibre.com/cerowrt/subscribe.html