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@xxxxxxxxxx>
---
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]);
