linux-kernel - Re: RT_GROUP_SCHED throttling blocks unthrottled RT tasks?

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Date:   Fri, 5 Nov 2021 19:05:41 -0400
From:   Joel Fernandes <joelaf@...gle.com>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: RT_GROUP_SCHED throttling blocks unthrottled RT tasks?

On Fri, Nov 5, 2021 at 7:04 PM Joel Fernandes <joelaf@...gle.com> wrote:
>
> On Fri, Nov 5, 2021 at 1:44 PM Doug Anderson <dianders@...omium.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

Errm, I meant Line 1060 in rt.c
https://elixir.bootlin.com/linux/latest/source/kernel/sched/rt.c#L1060