Re: [PATCH bpf-next] selftests: xsk: read current MAX_SKB_FRAGS from sysctl knob

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Tue, Sep 10, 2024 at 01:48:35PM +0200, Magnus Karlsson wrote:
> On Mon, 9 Sept 2024 at 16:12, Maciej Fijalkowski
> <maciej.fijalkowski@intel.com> wrote:
> >
> > Currently, xskxceiver assumes that MAX_SKB_FRAGS value is always 17
> > which is not true - since the introduction of BIG TCP this can now take
> > any value between 17 to 45 via CONFIG_MAX_SKB_FRAGS.
> >
> > Adjust the TOO_MANY_FRAGS test case to read the currently configured
> > MAX_SKB_FRAGS value by reading it from /proc/sys/net/core/max_skb_frags.
> >
> > Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> > ---
> >  tools/testing/selftests/bpf/xskxceiver.c | 41 +++++++++++++++++++++---
> >  tools/testing/selftests/bpf/xskxceiver.h |  1 -
> >  2 files changed, 36 insertions(+), 6 deletions(-)
> >
> > diff --git a/tools/testing/selftests/bpf/xskxceiver.c b/tools/testing/selftests/bpf/xskxceiver.c
> > index 92af633faea8..595b6da26897 100644
> > --- a/tools/testing/selftests/bpf/xskxceiver.c
> > +++ b/tools/testing/selftests/bpf/xskxceiver.c
> > @@ -325,6 +325,25 @@ static bool ifobj_zc_avail(struct ifobject *ifobject)
> >         return zc_avail;
> >  }
> >
> > +#define MAX_SKB_FRAGS_PATH "/proc/sys/net/core/max_skb_frags"
> > +static unsigned int get_max_skb_frags(void)
> > +{
> > +       unsigned int max_skb_frags = 0;
> > +       FILE *file;
> > +
> > +       file = fopen(MAX_SKB_FRAGS_PATH, "r");
> > +       if (!file) {
> > +               ksft_print_msg("Error opening %s\n", MAX_SKB_FRAGS_PATH);
> > +               return 0;
> > +       }
> > +
> > +       if (fscanf(file, "%u", &max_skb_frags) != 1)
> > +               ksft_print_msg("Error reading %s\n", MAX_SKB_FRAGS_PATH);
> > +
> > +       fclose(file);
> > +       return max_skb_frags;
> > +}
> > +
> >  static struct option long_options[] = {
> >         {"interface", required_argument, 0, 'i'},
> >         {"busy-poll", no_argument, 0, 'b'},
> > @@ -2245,13 +2264,22 @@ static int testapp_poll_rxq_tmout(struct test_spec *test)
> >
> >  static int testapp_too_many_frags(struct test_spec *test)
> >  {
> > -       struct pkt pkts[2 * XSK_DESC__MAX_SKB_FRAGS + 2] = {};
> > +       struct pkt *pkts;
> >         u32 max_frags, i;
> > +       int ret;
> >
> > -       if (test->mode == TEST_MODE_ZC)
> > +       if (test->mode == TEST_MODE_ZC) {
> >                 max_frags = test->ifobj_tx->xdp_zc_max_segs;
> > -       else
> > -               max_frags = XSK_DESC__MAX_SKB_FRAGS;
> > +       } else {
> > +               max_frags = get_max_skb_frags();
> > +               if (!max_frags)
> > +                       return TEST_FAILURE;
> 
> Thanks for this fix Maciej. However, I think failing the test here is
> a little bit too drastic. How about just returning TEST_SKIP and print
> out that the max number of skbs is unknown as the reason for the skip?
> Or even more optimistically, print out a warning that we could not
> read the max number of skb but we are guessing 17 and then run the
> test? If it passes, great we guessed correctly, but if it fails we are
> not worse off than the current code.

makes sense to default to 17 if we couldn't read it from file. will fix in
v2.

> Do not know how often a file
> system does not contain /proc/sys/net/core/max_skb_frags though.

A mix of CONFIG_NET, CONFIG_PROC_FS and CONFIG_SYSCTL i think.

> 
> > +               max_frags += 1;
> > +       }
> > +
> > +       pkts = calloc(2 * max_frags + 2, sizeof(struct pkt));
> > +       if (!pkts)
> > +               return TEST_FAILURE;
> >
> >         test->mtu = MAX_ETH_JUMBO_SIZE;
> >
> > @@ -2281,7 +2309,10 @@ static int testapp_too_many_frags(struct test_spec *test)
> >         pkts[2 * max_frags + 1].valid = true;
> >
> >         pkt_stream_generate_custom(test, pkts, 2 * max_frags + 2);
> > -       return testapp_validate_traffic(test);
> > +       ret = testapp_validate_traffic(test);
> > +
> > +       free(pkts);
> > +       return ret;
> >  }
> >
> >  static int xsk_load_xdp_programs(struct ifobject *ifobj)
> > diff --git a/tools/testing/selftests/bpf/xskxceiver.h b/tools/testing/selftests/bpf/xskxceiver.h
> > index 885c948c5d83..e46e823f6a1a 100644
> > --- a/tools/testing/selftests/bpf/xskxceiver.h
> > +++ b/tools/testing/selftests/bpf/xskxceiver.h
> > @@ -55,7 +55,6 @@
> >  #define XSK_UMEM__LARGE_FRAME_SIZE (3 * 1024)
> >  #define XSK_UMEM__MAX_FRAME_SIZE (4 * 1024)
> >  #define XSK_DESC__INVALID_OPTION (0xffff)
> > -#define XSK_DESC__MAX_SKB_FRAGS 18
> >  #define HUGEPAGE_SIZE (2 * 1024 * 1024)
> >  #define PKT_DUMP_NB_TO_PRINT 16
> >  #define RUN_ALL_TESTS UINT_MAX
> > --
> > 2.34.1
> >
> >