Re: [PATCH mptcp-next v3 0/3] send() fails with EAGAIN in blocking IO mode #487

[Mat Martineau <martineau@kernel.org>]

On Thu, 29 May 2025, Geliang Tang wrote:

> From: Geliang Tang <tanggeliang@kylinos.cn>
>
> Good news! I finally solved the unstable issue of MPTCP BPF sched selftests
> I reported a year ago, #487 "send() fails with EAGAIN in blocking IO mode".
>
> The fix is simple, it can be solved by explicitly setting SO_SNDBUF
> sockopt, but be sure not to set SO_RCVBUF at the same time
> (see sk->sk_userlocks & SOCK_RCVBUF_LOCK in mptcp_rcv_space_adjust()).
>
> With this fix, BPF sched selftests are now very stable, I run loop testing
> using mptcp-upstream-virtme-docker (run_loop run_bpftest_all), and can run
> it normally for hundreds of times without error:
>

Hi Geliang -

I can see how changing SO_SNDBUF on the sending socket side would shift 
timing behavior in a way that affect the test outcome, but it doesn't 
address the root issue with bug #487:

It is either OK to get an EAGAIN from a blocking send(), or it's not OK.


If it's not ok to ever return EAGAIN from a blocking send, the existing 
test code is a reproducer for a bug, and changing the test is hiding that 
bug.

If EAGAIN is ok, then we should change the code in send_recv_server() to 
allow it.


As Matthieu noted in the comment here 
(https://github.com/multipath-tcp/mptcp_net-next/issues/487#issuecomment-2083123666), 
it may be due to the BPF context that the socket is really non-blocking. 
Did we ever figure out if that was the case - I couldn't find the 
discussion on the mailing list?


- Mat


>
> 	=== Attempt: 465 (Thu, 29 May 2025 04:04:09 +0000) ===
>
> BPF Test: test_progs -t mptcp
> TAP version 13
> 1..1
> # #198/1   mptcp/base:OK
> # #198/2   mptcp/mptcpify:OK
> # #198/3   mptcp/subflow:OK
> # #198/4   mptcp/iters_subflow:OK
> # #198/5   mptcp/default:OK
> # #198/6   mptcp/first:OK
> # #198/7   mptcp/bkup:OK
> # #198/8   mptcp/rr:OK
> # #198/9   mptcp/red:OK
> # #198/10  mptcp/burst:OK
> # #198     mptcp:OK
> # Summary: 1/10 PASSED, 0 SKIPPED, 0 FAILED
> ok 1 test: bpftest_test_progs_mptcp
> # time=3
> BPF Test: test_progs-cpuv4 -t mptcp
> TAP version 13
> 1..1
> # #198/1   mptcp/base:OK
> # #198/2   mptcp/mptcpify:OK
> # #198/3   mptcp/subflow:OK
> # #198/4   mptcp/iters_subflow:OK
> # #198/5   mptcp/default:OK
> # #198/6   mptcp/first:OK
> # #198/7   mptcp/bkup:OK
> # #198/8   mptcp/rr:OK
> # #198/9   mptcp/red:OK
> # #198/10  mptcp/burst:OK
> # #198     mptcp:OK
> # Summary: 1/10 PASSED, 0 SKIPPED, 0 FAILED
> ok 1 test: bpftest_test_progs-cpuv4_mptcp
> # time=4
> BPF Test: test_progs-no_alu32 -t mptcp
> TAP version 13
> 1..1
> # #198/1   mptcp/base:OK
> # #198/2   mptcp/mptcpify:OK
> # #198/3   mptcp/subflow:OK
> # #198/4   mptcp/iters_subflow:OK
> # #198/5   mptcp/default:OK
> # #198/6   mptcp/first:OK
> # #198/7   mptcp/bkup:OK
> # #198/8   mptcp/rr:OK
> # #198/9   mptcp/red:OK
> # #198/10  mptcp/burst:OK
> # #198     mptcp:OK
> # Summary: 1/10 PASSED, 0 SKIPPED, 0 FAILED
> ok 1 test: bpftest_test_progs-no_alu32_mptcp
> # time=3
>
> 	=== Attempt: 466 (Thu, 29 May 2025 04:04:19 +0000) ===
>
>
> This set also invalidates the following set named "add io thread mode
> tests":
> https://patchwork.kernel.org/project/mptcp/cover/cover.1722502941.git.tanggeliang@kylinos.cn/
>
> v2:
> - mptcp: fix the default value of scaling_ratio
> https://patchwork.kernel.org/project/mptcp/patch/0ccc1c26d27d6ee7be22806a97983d37c6ca548c.1715053270.git.tanggeliang@kylinos.cn/
>
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/487
>
> Geliang Tang (3):
>  selftests/bpf: Add sndbuf for send_recv_data
>  Squash to "selftests/bpf: Add bpf scheduler test"
>  DO-NOT-MERGE: selftests/bpf: Increase total_bytes of bpf sched tests
>
> tools/testing/selftests/bpf/network_helpers.c | 20 ++++++++++++++++++-
> tools/testing/selftests/bpf/network_helpers.h |  2 +-
> .../selftests/bpf/prog_tests/bpf_qdisc.c      |  2 +-
> .../selftests/bpf/prog_tests/bpf_tcp_ca.c     |  4 ++--
> .../testing/selftests/bpf/prog_tests/mptcp.c  | 13 ++++++++++--
> 5 files changed, 34 insertions(+), 7 deletions(-)
>
> -- 
> 2.43.0
>
>
>