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From:   Joel Fernandes <joelaf@google.com>
Date:   Fri, 5 Nov 2021 19:04:00 -0400
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Subject: Re: RT_GROUP_SCHED throttling blocks unthrottled RT tasks?
To:     Doug Anderson <dianders@chromium.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, Nov 5, 2021 at 1:44 PM Doug Anderson <dianders@chromium.org> wrote:
[..]
>
>
> I tried gathering some tracing. One bit that might (?) be relevant:
>
>  cros_ec_spi_hig-179     [000] d.h5  1495.305919: sched_waking:
> comm=kworker/4:2 pid=5232 prio=120 target_cpu=004
>  cros_ec_spi_hig-179     [000] d.h6  1495.305926: sched_wakeup:
> comm=kworker/4:2 pid=5232 prio=120 target_cpu=004
>           <idle>-0       [001] d.H5  1495.309113: sched_waking:
> comm=sugov:6 pid=2658 prio=-1 target_cpu=006
>           <idle>-0       [001] d.H6  1495.309119: sched_wakeup:
> comm=sugov:6 pid=2658 prio=-1 target_cpu=006
>  cros_ec_spi_hig-179     [000] d.h5  1495.309336: sched_waking:
> comm=sugov:6 pid=2658 prio=-1 target_cpu=006
>  cros_ec_spi_hig-179     [000] d.h6  1495.309341: sched_wakeup:
> comm=sugov:6 pid=2658 prio=-1 target_cpu=006
>           <idle>-0       [001] d.H5  1495.312137: sched_waking:
> comm=sugov:6 pid=2658 prio=-1 target_cpu=006
>           <idle>-0       [001] d.H6  1495.312142: sched_wakeup:
> comm=sugov:6 pid=2658 prio=-1 target_cpu=006
>  cros_ec_spi_hig-179     [000] d.h5  1495.312859: sched_waking:
> comm=sugov:6 pid=2658 prio=-1 target_cpu=006
>  cros_ec_spi_hig-179     [000] d.h6  1495.312870: sched_wakeup:
> comm=sugov:6 pid=2658 prio=-1 target_cpu=006
>
> My best guess is that there's some bug in the scheduler where it just
> loops constantly picking an unthrottled RT task but then incorrectly
> decides that it's throttled and thus doesn't run it.

Thanks for posting this. Tricky bit indeed. I was wondering if the
issue is here:
https://elixir.bootlin.com/linux/latest/source/kernel/sched/rt.c#L1031

Basically, if something is actively getting RT-throttled on a CPU,
then if something else unthrottled but RT is ready to run, the
scheduler does resched_curr(rq) anyway. This probably happens when
update_curr_rt() is called on the unthrottled RT task via
task_tick_rt().    That seems a limitation of the current RT scheduler
I think? Thoughts?