Re: [LTP] [PATCH v1 1/1] sched_football: harden kickoff synchronization on high-CPU systems

[Andrea Cervesato via ltp <ltp@lists.linux.it>]

On Tue Feb 24, 2026 at 10:03 PM CET, John Stultz via ltp wrote:
> On Tue, Feb 24, 2026 at 2:45 AM Jan Polensky <japo@linux.ibm.com> wrote:
> >
> > The sched_football test has been intermittently failing, most noticeably
> > on systems with many CPUs and/or under load, due to a startup ordering
> > hole around kickoff.
> >
>
> I've not had time to closely review your suggestion here, but it
> sounds reasonable.
>
> That said, I want to warn you and ensure you are aware: the
> RT_PUSH_IPI feature in the scheduler breaks the RT invariant
> sched_football is testing.
>
> I see this as a bug with that feature, but the scalability RT_PUSH_IPI
> allows for seems more important to folks who are doing RT work then
> the mis-behavior.  Steven and I talked awhile back about some ideas on
> how we might be able to do the pull in a more efficient way while
> still holding the invariant true, and I have a bug to track it, but
> its not been high enough priority to get bandwidth yet.
>
> So you might want to make sure you disable that feature before testing via:
> # echo NO_RT_PUSH_IPI > /sys/kernel/debug/sched/features
>
> thanks
> -john

Thanks for your deep analysis on the possible issue. I'm not an RT expert,
but I trust your expertise in here :-) Will leave this patch review to
someone who's more skilled than me in this topic.

I have a suggestion tho.

About the NO_RT_PUSH_IPI, we are lucky: LTP provides a safe mechanism to
set the sys configurations and to restore it to default value after
test. You can find this in the `struct tst_test` and it's called
`.save_restore` [1]

I think it's worth to force this option according to the underlying
variant (and properly document this in the code with a comment).

WDYT?

[1] https://linux-test-project.readthedocs.io/en/latest/developers/api_c_tests.html#tst-test-definition
-- 
Andrea Cervesato
SUSE QE Automation Engineer Linux
andrea.cervesato@suse.com


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