[LTP] sched_football: Validity of testcase

[Joerg Vehlow]

Hi,

is no one interested in this?

> Hi,
> 
> I was looking thoroughly at the realtime testcase sched_football,
> because it sometimes fails and like to know your opinion on the test case.
> 
> A short introduction to how the test works:
> It creates nThreads threads called offense and n threads called defense
> (all fifo scheduled). The offense threads run at a lower priority than
> the defense threads and the main thread has the highest priority. After
> all threads are created (validated using an atomic counter). The test
> verifies, that the offense threads are never executed by incrementing
> a counter in the offense threads, that is zeroed in the main thread.
> During the test the main threads sleeps to regularly.
> 
> While the test is totally fine on a single core system, you can
> immediately see, that it will fail on a system with nCores > nThreads,
> because there will be a core were only an offense thread an no defense
> thread is scheduled. In its default setup nThreads = nCores. This should
> theoretically work, because there is a defense thread for every score with
> 
> a higher priority than the offense threads and they should be scheduled
> onto  every core. This is indeed what happens. The problem seems to be
> the  initialization phase. When the threads are created, they are not
> evenly scheduled. After pthread_create was called, the threads are
> scheduled
> 
> too cores where nothing is running. If there is no idle core anymore, they
> are
> scheduled to any core (the first?, the one with the shortest wait queue?).
> At
> some point after all threads are created, they are rescheduled to every
> core.
> It looks like the test fails, when there is initially a core with only an
> offense thread scheduled onto it. In perf sched traces I saw, that a
> defense
> thread was migrated to this core, but still the offense thread was
> executed
> for
> a short time, until the offense thread runs. From this point onwards only
> defense threads are running.
> 
> I tested adding a sleep to the main function, after all threads are
> created,
> to give the system some time for rescheduling. A sleep of around 50ms
> works
> quite well and supports my theory about the migration time being the
> problem.
> 
> Now I am not sure if the test case is even valid or if the scheduler is
> not
> working as it is supposed to. Looking at the commits of sched_football it
> looks like it was running stable at least at some point, at least it es
> reported to have run 15k iterations in e6432e45.
> What do you think about the test case? Is it even valid?
> Should the cpu affinity be set fixed?
> 
> A note about my testing methodology:
> After I realized, that the execution often failed due to the offense
> thread
> running after referee set the_ball to 0, I replaced the loop with just
> usleep(10000), for faster iteration.
> I tested on ubuntu 19.04 with linux 5.0.0-27 running in vmware and
> a custom yocto distribution running linux 4.19.59 (with and without rt
> patches)
> 
> Jörg