netdev - Re: [PATCH bpf] selftests/bpf: work-around EBUSY errors from hashmap update/delete

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Date:   Tue, 22 Dec 2020 16:04:32 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf] selftests/bpf: work-around EBUSY errors from hashmap update/delete

On Tue, Dec 22, 2020 at 3:58 PM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Dec 22, 2020, at 11:53 AM, Andrii Nakryiko <andrii@...nel.org> wrote:
> >
> > 20b6cc34ea74 ("bpf: Avoid hashtab deadlock with map_locked") introduced
> > a possibility of getting EBUSY error on lock contention, which seems to happen
> > very deterministically in test_maps when running 1024 threads on low-CPU
> > machine. In libbpf CI case, it's a 2 CPU VM and it's hitting this 100% of the
> > time. Work around by retrying on EBUSY (and EAGAIN, while we are at it) after
> > a small sleep. sched_yield() is too agressive and fails even after 20 retries,
> > so I went with usleep(1) for backoff.
> >
> > Also log actual error returned to make it easier to see what's going on.
> >
> > Fixes: 20b6cc34ea74 ("bpf: Avoid hashtab deadlock with map_locked")
> > Cc: Song Liu <songliubraving@...com>
> > Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
>
> Thanks for the fix!
>
> Acked-by: Song Liu <songliubraving@...com>
>
> With one minor nitpick below
>
> > ---
> > tools/testing/selftests/bpf/test_maps.c | 46 +++++++++++++++++++++----
> > 1 file changed, 40 insertions(+), 6 deletions(-)
> >
> > diff --git a/tools/testing/selftests/bpf/test_maps.c b/tools/testing/selftests/bpf/test_maps.c
> > index 0ad3e6305ff0..809004f4995f 100644
> > --- a/tools/testing/selftests/bpf/test_maps.c
> > +++ b/tools/testing/selftests/bpf/test_maps.c
> > @@ -1312,22 +1312,56 @@ static void test_map_stress(void)
> > #define DO_UPDATE 1
> > #define DO_DELETE 0
>
> [...]
>
> > +                             printf("error %d %d\n", err, errno);
> > +                     assert(err == 0);
> > +                     err = map_update_retriable(fd, &key, &value, BPF_EXIST, 20);
> > +                     if (err)
> > +                             printf("error %d %d\n", err, errno);
> > +                     assert(err == 0);
> >               } else {
> > -                     assert(bpf_map_delete_elem(fd, &key) == 0);
> > +                     err = map_delete_retriable(fd, &key, 5);
>
> nit: Why 5 here vs. 20 above?

Forgot to update here. I'll make all of them the same, thanks.

>
> > +                     if (err)
> > +                             printf("error %d %d\n", err, errno);
> > +                     assert(err == 0);
> >               }
> >       }
> > }
> > --
> > 2.24.1
> >
>