[LTP] [PATCH v2] sched_football: synchronize with kickoff flag to reduce skew

[Li Wang]

On Thu, Sep 4, 2025 at 7:42 PM Cyril Hrubis <chrubis@suse.cz> wrote:

> Hi!
> > >  static void do_setup(void)
> > >  {
> > > +   if (!tst_check_preempt_rt())
> > > +           tst_brk(TCONF, "Test requires real-time kernel");
> >
> > I understood Cyril is really suggesting to keep it [1]. I would also
> vote to
> > keep it (we still have some time to see if it got fixed before release).
> >
> > I know we had this discussion in the past (some of your colleague
> suggesting it
> > should not be run on non-RT kernel), so I'm not pushing for it.
>
> I stil do not understand reasons for disabling the test. The POSIX
> realtime schedulling classes have to work properly regardless of the
> kernel flavor. Why should we turn the test off on non-rt kernel then?
>

No special reasons. I still can sporadically catch the failure on non-RT
kernel with even sleep 2 seconds.

Thus, I took this very extreme approach, because on non-RT kernel, sleep
may not have a particularly perfect effect, I guess the stock kernel with
sched_setscheduler(, SCHED_FIFO, ) is still has scheduling skew with
workload.

But the RT kernel is more aggressive in scheduling, and it is almost
impossible
to encounter false positives with that enhancement.


--- a/testcases/realtime/func/sched_football/sched_football.c
+++ b/testcases/realtime/func/sched_football/sched_football.c
@@ -130,6 +130,8 @@ void referee(int game_length)
        pthread_barrier_wait(&start_barrier);
        tst_atomic_store(0, &the_ball);
        tst_atomic_store(1, &kickoff_flag);
+       usleep(2000000);
+

        /* Watch the game */
        while ((now.tv_sec - start.tv_sec) < game_length) {
@@ -199,9 +201,6 @@ static void do_test(void)

 static void do_setup(void)
 {
-       if (!tst_check_preempt_rt())
-               tst_brk(TCONF, "Test requires real-time kernel");
-
        if (tst_parse_int(str_game_length, &game_length, 1, INT_MAX))
                tst_brk(TBROK, "Invalid game length '%s'", str_game_length);

# ./sched_football
tst_test.c:2004: TINFO: LTP version: 20250530
tst_test.c:2007: TINFO: Tested kernel: 6.12.0-55.29.1.el10_0.x86_64
                                  #1 SMP PREEMPT_DYNAMIC Mon Aug 18
05:37:24 EDT 2025 x86_64
tst_kconfig.c:88: TINFO: Parsing kernel config
'/lib/modules/6.12.0-55.29.1.el10_0.x86_64/config'
tst_test.c:1825: TINFO: Overall timeout per run is 0h 00m 30s
sched_football.c:162: TINFO: players_per_team: 32 game_length: 5
sched_football.c:178: TINFO: Starting 32 offense threads at priority 15
sched_football.c:185: TINFO: Starting 32 defense threads at priority 30
sched_football.c:192: TINFO: Starting 64 crazy-fan threads at priority 50
sched_football.c:118: TINFO: Starting referee thread
sched_football.c:121: TINFO: Starting the game (5 sec)
sched_football.c:144: TINFO: Final ball position: 16761
sched_football.c:150: TFAIL: Expect: final_ball == 0



>
> It may make sense to prolong the settling period for non-rt something
> as:
>
> ...
>         if (tst_check_preempt_rt()
>                 settling_period = 20000;
>         else
>                 settling_period = 200000;
>
>
> ...
>         usleep(settling_period);
> ...
>
>
> In order to make sure non-rt scheduller has enough time to shuffle the
> processes around the available CPUs. But that should be the only
> difference.
>
> --
> Cyril Hrubis
> chrubis@suse.cz
>
>

-- 
Regards,
Li Wang