bpf: Introduce SK_BPF_BYPASS_PROT_MEM.

If a socket has sk->sk_bypass_prot_mem flagged, the socket opts out
of the global protocol memory accounting.

This is easily controlled by net.core.bypass_prot_mem sysctl, but it
lacks flexibility.

Let's support flagging (and clearing) sk->sk_bypass_prot_mem via
bpf_setsockopt() at the BPF_CGROUP_INET_SOCK_CREATE hook.

  int val = 1;

  bpf_setsockopt(ctx, SOL_SOCKET, SK_BPF_BYPASS_PROT_MEM,
                 &val, sizeof(val));

As with net.core.bypass_prot_mem, this is inherited to child sockets,
and BPF always takes precedence over sysctl at socket(2) and accept(2).

SK_BPF_BYPASS_PROT_MEM is only supported at BPF_CGROUP_INET_SOCK_CREATE
and not supported on other hooks for some reasons:

  1. UDP charges memory under sk->sk_receive_queue.lock instead
     of lock_sock()

  2. Modifying the flag after skb is charged to sk requires such
     adjustment during bpf_setsockopt() and complicates the logic
     unnecessarily

We can support other hooks later if a real use case justifies that.

Most changes are inline and hard to trace, but a microbenchmark on
__sk_mem_raise_allocated() during neper/tcp_stream showed that more
samples completed faster with sk->sk_bypass_prot_mem == 1.  This will
be more visible under tcp_mem pressure (but it's not a fair comparison).

  # bpftrace -e 'kprobe:__sk_mem_raise_allocated { @start[tid] = nsecs; }
    kretprobe:__sk_mem_raise_allocated /@start[tid]/
    { @end[tid] = nsecs - @start[tid]; @times = hist(@end[tid]); delete(@start[tid]); }'
  # tcp_stream -6 -F 1000 -N -T 256

Without bpf prog:

  [128, 256)          3846 |                                                    |
  [256, 512)       1505326 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
  [512, 1K)        1371006 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@     |
  [1K, 2K)          198207 |@@@@@@                                              |
  [2K, 4K)           31199 |@                                                   |

With bpf prog in the next patch:
  (must be attached before tcp_stream)
  # bpftool prog load sk_bypass_prot_mem.bpf.o /sys/fs/bpf/test type cgroup/sock_create
  # bpftool cgroup attach /sys/fs/cgroup/test cgroup_inet_sock_create pinned /sys/fs/bpf/test

  [128, 256)          6413 |                                                    |
  [256, 512)       1868425 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
  [512, 1K)        1101697 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@                      |
  [1K, 2K)          117031 |@@@@                                                |
  [2K, 4K)           11773 |                                                    |

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Link: https://patch.msgid.link/20251014235604.3057003-6-kuniyu@google.com

1 files changed, 1 insertions, 0 deletions

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 6829936d33f5..9b17d937edf7 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -7200,6 +7200,7 @@ enum {
 	TCP_BPF_SYN_MAC         = 1007, /* Copy the MAC, IP[46], and TCP header */
 	TCP_BPF_SOCK_OPS_CB_FLAGS = 1008, /* Get or Set TCP sock ops flags */
 	SK_BPF_CB_FLAGS		= 1009, /* Get or set sock ops flags in socket */
+	SK_BPF_BYPASS_PROT_MEM	= 1010, /* Get or Set sk->sk_bypass_prot_mem */
 };
 
 enum {