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

[Zhenzhong Wu]

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@xxxxxxxxxx> wrote:
>
> From: Zhenzhong Wu <jt26wzz@xxxxxxxxx>
> 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@xxxxxxxxx>
> > ---
> >  .../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@xxxxxxxxxxxx>
> >   */
> >
> >  #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<---