From 1cface552a5b5f6e53a855de1a503ff958e2e253 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Thu, 7 Mar 2024 12:34:46 +0000 Subject: net: add skb_data_unref() helper Similar to skb_unref(), add skb_data_unref() to save an expensive atomic operation (and cache line dirtying) when last reference on shinfo->dataref is released. I saw this opportunity on hosts with RAW sockets accidentally bound to UDP protocol, forcing an skb_clone() on all received packets. These RAW sockets had their receive queue full, so all clone packets were immediately dropped. When UDP recvmsg() consumes later the original skb, skb_release_data() is hitting atomic_sub_return() quite badly, because skb->clone has been set permanently. Note that this patch helps TCP TX performance, because TCP stack also use (fast) clones. This means that at least one of the two packets (the main skb or its clone) will no longer have to perform this atomic operation in skb_release_data(). Signed-off-by: Eric Dumazet Link: https://lore.kernel.org/r/20240307123446.2302230-1-edumazet@google.com Signed-off-by: Jakub Kicinski --- include/linux/skbuff.h | 18 ++++++++++++++++++ net/core/skbuff.c | 4 +--- 2 files changed, 19 insertions(+), 3 deletions(-) diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index d0508f90bed5..3023bc2be6a1 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -1237,6 +1237,24 @@ static inline bool skb_unref(struct sk_buff *skb) return true; } +static inline bool skb_data_unref(const struct sk_buff *skb, + struct skb_shared_info *shinfo) +{ + int bias; + + if (!skb->cloned) + return true; + + bias = skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1; + + if (atomic_read(&shinfo->dataref) == bias) + smp_rmb(); + else if (atomic_sub_return(bias, &shinfo->dataref)) + return false; + + return true; +} + void __fix_address kfree_skb_reason(struct sk_buff *skb, enum skb_drop_reason reason); diff --git a/net/core/skbuff.c b/net/core/skbuff.c index 766219011aea..b99127712e67 100644 --- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -1115,9 +1115,7 @@ static void skb_release_data(struct sk_buff *skb, enum skb_drop_reason reason, struct skb_shared_info *shinfo = skb_shinfo(skb); int i; - if (skb->cloned && - atomic_sub_return(skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1, - &shinfo->dataref)) + if (!skb_data_unref(skb, shinfo)) goto exit; if (skb_zcopy(skb)) { -- cgit