netdev - [PATCH bpf-next V2] bpf/xdp: optimize bpf_xdp

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Message-ID: <168563651438.3436004.17735707525651776648.stgit@firesoul>
Date: Thu, 01 Jun 2023 18:21:54 +0200
From: Jesper Dangaard Brouer <brouer@...hat.com>
To: Tariq Toukan <ttoukan.linux@...il.com>,
 Daniel Borkmann <borkmann@...earbox.net>,
 Alexei Starovoitov <ast@...nel.org>,
 Andrii Nakryiko <andrii.nakryiko@...il.com>, bpf@...r.kernel.org
Cc: Jesper Dangaard Brouer <brouer@...hat.com>,
 Tariq Toukan <tariqt@...dia.com>, gal@...dia.com, lorenzo@...nel.org,
 netdev@...r.kernel.org, echaudro@...hat.com, andrew.gospodarek@...adcom.com
Subject: [PATCH bpf-next V2] bpf/xdp: optimize bpf_xdp_pointer to avoid
 reading sinfo

Currently we observed a significant performance degradation in
samples/bpf xdp1 and xdp2, due XDP multibuffer "xdp.frags" handling,
added in commit 772251742262 ("samples/bpf: fixup some tools to be able
to support xdp multibuffer").

This patch reduce the overhead by avoiding to read/load shared_info
(sinfo) memory area, when XDP packet don't have any frags. This improves
performance because sinfo is located in another cacheline.

Function bpf_xdp_pointer() is used by BPF helpers bpf_xdp_load_bytes()
and bpf_xdp_store_bytes(). As a help to reviewers, xdp_get_buff_len() can
potentially access sinfo, but it uses xdp_buff_has_frags() flags bit check
to avoid accessing sinfo in no-frags case.

The likely/unlikely instrumentation lays out asm code such that sinfo
access isn't interleaved with no-frags case (checked on GCC 12.2.1-4).
The generated asm code is more compact towards the no-frags case.

The BPF kfunc bpf_dynptr_slice() also use bpf_xdp_pointer(). Thus, it
should also take effect for that.

Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
---
 net/core/filter.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 968139f4a1ac..961db5bd2f94 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3948,20 +3948,21 @@ void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off,

 void *bpf_xdp_pointer(struct xdp_buff *xdp, u32 offset, u32 len)
 {
-	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
 	u32 size = xdp->data_end - xdp->data;
+	struct skb_shared_info *sinfo;
 	void *addr = xdp->data;
 	int i;

 	if (unlikely(offset > 0xffff || len > 0xffff))
 		return ERR_PTR(-EFAULT);

-	if (offset + len > xdp_get_buff_len(xdp))
+	if (unlikely(offset + len > xdp_get_buff_len(xdp)))
 		return ERR_PTR(-EINVAL);

-	if (offset < size) /* linear area */
+	if (likely((offset < size))) /* linear area */
 		goto out;

+	sinfo = xdp_get_shared_info_from_buff(xdp);
 	offset -= size;
 	for (i = 0; i < sinfo->nr_frags; i++) { /* paged area */
 		u32 frag_size = skb_frag_size(&sinfo->frags[i]);