Re: [PATCH net v2 2/2] selftests/bpf: check epoll readiness after reuseport migration

Zhenzhong Wu
2026-04-21T11:16:17Z

Thanks Kuniyuki, will fold this into v3.

Your approach of checking epoll right around shutdown() is much
better. I didn't think to put the check inside migrate_dance(). With
my original placement, the test still passed without patch 1 — I
should have called this out in the cover letter instead of settling
for what was effectively just a smoke test.

On Tue, Apr 21, 2026 at 3:20 PM Kuniyuki Iwashima <kuniyu@google.com> wrote:
>
> From: Zhenzhong Wu <jt26wzz@gmail.com>
> Date: Sun, 19 Apr 2026 02:13:33 +0800
> > After migrate_dance() moves established children to the target
> > listener, add it to an epoll set and verify that epoll_wait(..., 0)
> > reports it ready before accept().
> >
> > This adds epoll coverage for the TCP_ESTABLISHED reuseport migration
> > case in migrate_reuseport.
> >
> > Keep the check limited to TCP_ESTABLISHED cases. TCP_SYN_RECV and
> > TCP_NEW_SYN_RECV still depend on asynchronous handshake completion,
> > so a zero-timeout epoll_wait() would race there.
> >
> > Signed-off-by: Zhenzhong Wu <jt26wzz@gmail.com>
> > ---
> > .../bpf/prog_tests/migrate_reuseport.c | 32 ++++++++++++++++++-
> > 1 file changed, 31 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c b/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
> > index 653b0a20f..580a53424 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
> > @@ -18,13 +18,16 @@
> > * 9. call shutdown() for the second server
> > * and migrate the requests in the accept queue
> > * to the last server socket.
> > - * 10. call accept() for the last server socket.
> > + * 10. for TCP_ESTABLISHED cases, call epoll_wait(..., 0)
> > + * for the last server socket.
> > + * 11. call accept() for the last server socket.
> > *
> > * Author: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
> > */
> >
> > #include <bpf/bpf.h>
> > #include <bpf/libbpf.h>
> > +#include <sys/epoll.h>
> >
> > #include "test_progs.h"
> > #include "test_migrate_reuseport.skel.h"
> > @@ -522,6 +525,33 @@ static void run_test(struct migrate_reuseport_test_case *test_case,
> > goto close_clients;
> > }
> >
> > + /* Only TCP_ESTABLISHED has already-migrated accept-queue entries
> > + * here. Later states still depend on follow-up handshake work.
> > + */
> > + if (test_case->state == BPF_TCP_ESTABLISHED) {
> > + struct epoll_event ev = {
> > + .events = EPOLLIN,
> > + };
> > + int epfd;
> > + int nfds;
> > +
> > + epfd = epoll_create1(EPOLL_CLOEXEC);
> > + if (!ASSERT_NEQ(epfd, -1, "epoll_create1"))
> > + goto close_clients;
> > +
> > + ev.data.fd = test_case->servers[MIGRATED_TO];
> > + if (!ASSERT_OK(epoll_ctl(epfd, EPOLL_CTL_ADD,
> > + test_case->servers[MIGRATED_TO], &ev),
> > + "epoll_ctl"))
> > + goto close_epfd;
> > +
> > + nfds = epoll_wait(epfd, &ev, 1, 0);
> > + ASSERT_EQ(nfds, 1, "epoll_wait");
>
> Thanks for the update, but the test passes without patch 1.
>
> I think it would be best to test just after shutdown()
> where migration happens.
>
> Also, TCP_SYN_RECV should be covered in the same way.
>
> ---8<---
> diff --git a/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c b/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
> index 580a534249a7..66fea936649e 100644
> --- a/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
> +++ b/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
> @@ -353,8 +353,29 @@ static int update_maps(struct migrate_reuseport_test_case *test_case,
>
> static int migrate_dance(struct migrate_reuseport_test_case *test_case)
> {
> + struct epoll_event ev = {
> + .events = EPOLLIN,
> + };
> + int epoll, nfds;
> int i, err;
>
> + if (test_case->state != BPF_TCP_NEW_SYN_RECV) {
> + epoll = epoll_create1(0);
> + if (!ASSERT_NEQ(epoll, -1, "epoll_create1"))
> + return -1;
> +
> + ev.data.fd = test_case->servers[MIGRATED_TO];
> + if (!ASSERT_OK(epoll_ctl(epoll, EPOLL_CTL_ADD,
> + test_case->servers[MIGRATED_TO], &ev),
> + "epoll_ctl")) {
> + goto close_epoll;
> + }
> +
> + nfds = epoll_wait(epoll, &ev, 1, 0);
> + if (!ASSERT_EQ(nfds, 0, "epoll_wait 1"))
> + goto close_epoll;
> + }
> +
> /* Migrate TCP_ESTABLISHED and TCP_SYN_RECV requests
> * to the last listener based on eBPF.
> */
> @@ -368,6 +389,15 @@ static int migrate_dance(struct migrate_reuseport_test_case *test_case)
> if (test_case->state == BPF_TCP_NEW_SYN_RECV)
> return 0;
>
> + nfds = epoll_wait(epoll, &ev, 1, 0);
> + if (!ASSERT_EQ(nfds, 1, "epoll_wait 2")) {
> +close_epoll:
> + close(epoll);
> + return -1;
> + }
> +
> + close(epoll);
> +
> /* Note that we use the second listener instead of the
> * first one here.
> *
> @@ -525,33 +555,6 @@ static void run_test(struct migrate_reuseport_test_case *test_case,
> goto close_clients;
> }
>
> - /* Only TCP_ESTABLISHED has already-migrated accept-queue entries
> - * here. Later states still depend on follow-up handshake work.
> - */
> - if (test_case->state == BPF_TCP_ESTABLISHED) {
> - struct epoll_event ev = {
> - .events = EPOLLIN,
> - };
> - int epfd;
> - int nfds;
> -
> - epfd = epoll_create1(EPOLL_CLOEXEC);
> - if (!ASSERT_NEQ(epfd, -1, "epoll_create1"))
> - goto close_clients;
> -
> - ev.data.fd = test_case->servers[MIGRATED_TO];
> - if (!ASSERT_OK(epoll_ctl(epfd, EPOLL_CTL_ADD,
> - test_case->servers[MIGRATED_TO], &ev),
> - "epoll_ctl"))
> - goto close_epfd;
> -
> - nfds = epoll_wait(epfd, &ev, 1, 0);
> - ASSERT_EQ(nfds, 1, "epoll_wait");
> -
> -close_epfd:
> - close(epfd);
> - }
> -
> count_requests(test_case, skel);
>
> close_clients:
> ---8<---