On 31/05/2023 18.24, Toke Høiland-Jørgensen wrote:
Lorenzo Bianconi <lorenzo@xxxxxxxxxx> writes:
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.
Perf report show bpf_xdp_pointer() percentage utilization being reduced
from 4,19% to 3,37% (on CPU E5-1650 @3.60GHz).
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 | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
diff --git a/net/core/filter.c b/net/core/filter.c
index 968139f4a1ac..a635f537d499 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3948,20 +3948,24 @@ 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))
- return ERR_PTR(-EINVAL);
+ if (likely((offset < size))) /* linear area */
+ goto out;
Hi Jesper,
please correct me if I am wrong but looking at the code, in this way
bpf_xdp_pointer() will return NULL (and not ERR_PTR(-EINVAL)) if:
- offset < size
- offset + len > xdp_get_buff_len()
doing so I would say bpf_xdp_copy_buf() will copy the full packet starting from
offset leaving some part of the auxiliary buffer possible uninitialized.
Do you think it is an issue?
Yeah, you're right, bpf_xdp_load_bytes() should fail if trying to read
beyond the frame, and in this case it won't for non-frags; that's a
change in behaviour we probably shouldn't be making...
Thanks for spotting this!
I will work on a V2 tomorrow.
--Jesper
