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bpf: Allow opt-out from sk->sk_prot->memory_allocated. #9948

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subject: bpf: Allow opt-out from sk->sk_prot->memory_allocated.
version: 1
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1008847

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Upstream branch: 4c97c4b
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q2ven added 6 commits October 11, 2025 18:34
@q2ven
tcp: Save lock_sock() for memcg in inet_csk_accept().
ca17651 
If memcg is enabled, accept() acquires lock_sock() twice for each new
TCP/MPTCP socket in inet_csk_accept() and __inet_accept().

Let's move memcg operations from inet_csk_accept() to __inet_accept().

Note that SCTP somehow allocates a new socket by sk_alloc() in
sk->sk_prot->accept() and clones fields manually, instead of using
sk_clone_lock().

mem_cgroup_sk_alloc() is called for SCTP before __inet_accept(),
so I added the protocol check in __inet_accept(), but this can be
removed once SCTP uses sk_clone_lock().

Signed-off-by: Kuniyuki Iwashima <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
@q2ven
net: Allow opt-out from global protocol memory accounting.
6a7f33e 
Some protocols (e.g., TCP, UDP) implement memory accounting for socket
buffers and charge memory to per-protocol global counters pointed to by
sk->sk_proto->memory_allocated.

Sometimes, system processes do not want that limitation.  For a similar
purpose, there is SO_RESERVE_MEM for sockets under memcg.

Also, by opting out of the per-protocol accounting, sockets under memcg
can avoid paying costs for two orthogonal memory accounting mechanisms.
A microbenchmark result is in the subsequent bpf patch.

Let's allow opt-out from the per-protocol memory accounting if
sk->sk_bypass_prot_mem is true.

sk->sk_bypass_prot_mem and sk->sk_prot are placed in the same cache
line, and sk_has_account() always fetches sk->sk_prot before accessing
sk->sk_bypass_prot_mem, so there is no extra cache miss for this patch.

The following patches will set sk->sk_bypass_prot_mem to true, and
then, the per-protocol memory accounting will be skipped.

Note that this does NOT disable memcg, but rather the per-protocol one.

Another option not to use the hole in struct sock_common is create
sk_prot variants like tcp_prot_bypass, but this would complicate
SOCKMAP logic, tcp_bpf_prots etc.

Signed-off-by: Kuniyuki Iwashima <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
@q2ven
net: Introduce net.core.bypass_prot_mem sysctl.
d8e0205 
If a socket has sk->sk_bypass_prot_mem flagged, the socket opts out
of the global protocol memory accounting.

Let's control the flag by a new sysctl knob.

The flag is written once during socket(2) and is inherited to child
sockets.

Tested with a script that creates local socket pairs and send()s a
bunch of data without recv()ing.

Setup:

  # mkdir /sys/fs/cgroup/test
  # echo $$ >> /sys/fs/cgroup/test/cgroup.procs
  # sysctl -q net.ipv4.tcp_mem="1000 1000 1000"
  # ulimit -n 524288

Without net.core.bypass_prot_mem, charged to tcp_mem & memcg

  # python3 pressure.py &
  # cat /sys/fs/cgroup/test/memory.stat | grep sock
  sock 22642688 <-------------------------------------- charged to memcg
  # cat /proc/net/sockstat| grep TCP
  TCP: inuse 2006 orphan 0 tw 0 alloc 2008 mem 5376 <-- charged to tcp_mem
  # ss -tn | head -n 5
  State Recv-Q Send-Q Local Address:Port  Peer Address:Port
  ESTAB 2000   0          127.0.0.1:34479    127.0.0.1:53188
  ESTAB 2000   0          127.0.0.1:34479    127.0.0.1:49972
  ESTAB 2000   0          127.0.0.1:34479    127.0.0.1:53868
  ESTAB 2000   0          127.0.0.1:34479    127.0.0.1:53554
  # nstat | grep Pressure || echo no pressure
  TcpExtTCPMemoryPressures        1                  0.0

With net.core.bypass_prot_mem=1, charged to memcg only:

  # sysctl -q net.core.bypass_prot_mem=1
  # python3 pressure.py &
  # cat /sys/fs/cgroup/test/memory.stat | grep sock
  sock 2757468160 <------------------------------------ charged to memcg
  # cat /proc/net/sockstat | grep TCP
  TCP: inuse 2006 orphan 0 tw 0 alloc 2008 mem 0 <- NOT charged to tcp_mem
  # ss -tn | head -n 5
  State Recv-Q Send-Q  Local Address:Port  Peer Address:Port
  ESTAB 111000 0           127.0.0.1:36019    127.0.0.1:49026
  ESTAB 110000 0           127.0.0.1:36019    127.0.0.1:45630
  ESTAB 110000 0           127.0.0.1:36019    127.0.0.1:44870
  ESTAB 111000 0           127.0.0.1:36019    127.0.0.1:45274
  # nstat | grep Pressure || echo no pressure
  no pressure

Signed-off-by: Kuniyuki Iwashima <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
@q2ven
bpf: Support bpf_setsockopt() for BPF_CGROUP_INET_SOCK_CREATE.
c502126 
We will support flagging sk->sk_bypass_prot_mem via bpf_setsockopt()
at the BPF_CGROUP_INET_SOCK_CREATE hook.

BPF_CGROUP_INET_SOCK_CREATE is invoked by __cgroup_bpf_run_filter_sk()
that passes a pointer to struct sock to the bpf prog as void *ctx.

But there are no bpf_func_proto for bpf_setsockopt() that receives
the ctx as a pointer to struct sock.

Also, bpf_getsockopt() will be necessary for a cgroup with multiple
bpf progs running.

Let's add new bpf_setsockopt() and bpf_getsockopt() variants for
BPF_CGROUP_INET_SOCK_CREATE.

Note that inet_create() is not under lock_sock() and has the same
semantics with bpf_lsm_unlocked_sockopt_hooks.

Signed-off-by: Kuniyuki Iwashima <[email protected]>
@q2ven
bpf: Introduce SK_BPF_BYPASS_PROT_MEM.
f67efcc 
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 <[email protected]>
@q2ven
selftest: bpf: Add test for sk->sk_bypass_prot_mem.
355f612 
The test does the following for IPv4/IPv6 x TCP/UDP sockets
with/without sk->sk_bypass_prot_mem, which can be turned on by
net.core.bypass_prot_mem or bpf_setsockopt(SK_BPF_BYPASS_PROT_MEM).

  1. Create socket pairs
  2. Send NR_PAGES (32) of data (TCP consumes around 35 pages,
     and UDP consuems 66 pages due to skb overhead)
  3. Read memory_allocated from sk->sk_prot->memory_allocated and
     sk->sk_prot->memory_per_cpu_fw_alloc
  4. Check if unread data is charged to memory_allocated

If sk->sk_bypass_prot_mem is set, memory_allocated should not be
changed, but we allow a small error (up to 10 pages) in case
other processes on the host use some amounts of TCP/UDP memory.

The amount of allocated pages are buffered to per-cpu variable
{tcp,udp}_memory_per_cpu_fw_alloc up to +/- net.core.mem_pcpu_rsv
before reported to {tcp,udp}_memory_allocated.

At 3., memory_allocated is calculated from the 2 variables at
fentry of socket create function.

We drain the receive queue only for UDP before close() because UDP
recv queue is destroyed after RCU grace period.  When I printed
memory_allocated, UDP bypass cases sometimes saw the no-bypass
case's leftover, but it's still in the small error range (<10 pages).

  bpf_trace_printk: memory_allocated: 0   <-- TCP no-bypass
  bpf_trace_printk: memory_allocated: 35
  bpf_trace_printk: memory_allocated: 0   <-- TCP w/ sysctl
  bpf_trace_printk: memory_allocated: 0
  bpf_trace_printk: memory_allocated: 0   <-- TCP w/ bpf
  bpf_trace_printk: memory_allocated: 0
  bpf_trace_printk: memory_allocated: 0   <-- UDP no-bypass
  bpf_trace_printk: memory_allocated: 66
  bpf_trace_printk: memory_allocated: 2   <-- UDP w/ sysctl (2 pages leftover)
  bpf_trace_printk: memory_allocated: 2
  bpf_trace_printk: memory_allocated: 2   <-- UDP w/ bpf (2 pages leftover)
  bpf_trace_printk: memory_allocated: 2

We prefer finishing tests faster than oversleeping for call_rcu()
 + sk_destruct().

The test completes within 2s on QEMU (64 CPUs) w/ KVM.

  # time ./test_progs -t sk_bypass
  #371/1   sk_bypass_prot_mem/TCP  :OK
  #371/2   sk_bypass_prot_mem/UDP  :OK
  #371/3   sk_bypass_prot_mem/TCPv6:OK
  #371/4   sk_bypass_prot_mem/UDPv6:OK
  #371     sk_bypass_prot_mem:OK
  Summary: 1/4 PASSED, 0 SKIPPED, 0 FAILED

  real	0m1.481s
  user	0m0.181s
  sys	0m0.441s

Signed-off-by: Kuniyuki Iwashima <[email protected]>
@kernel-patches-daemon-bpf
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Upstream branch: 39e9d5f
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1008847
version: 1

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