Re: [PATCH] sched/rt: Disable RT_RUNTIME_SHARE by default

[Wei Wang]

On Tue, Sep 22, 2020 at 12:04 PM Daniel Bristot de Oliveira
<bristot@xxxxxxxxxx> wrote:
>
> On 9/22/20 7:14 PM, Wei Wang wrote:
> > On Mon, Sep 21, 2020 at 7:40 AM Daniel Bristot de Oliveira
> > <bristot@xxxxxxxxxx> wrote:
> >>
> >> The RT_RUNTIME_SHARE sched feature enables the sharing of rt_runtime
> >> between CPUs, allowing a CPU to run a real-time task up to 100% of the
> >> time while leaving more space for non-real-time tasks to run on the CPU
> >> that lend rt_runtime.
> >>
> >> The problem is that a CPU can easily borrow enough rt_runtime to allow
> >> a spinning rt-task to run forever, starving per-cpu tasks like kworkers,
> >> which are non-real-time by design.
> >>
> >> This patch disables RT_RUNTIME_SHARE by default, avoiding this problem.
> >> The feature will still be present for users that want to enable it,
> >> though.
> >>
> >> Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> >> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> >> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> >> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> >> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> >> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> >> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> >> Cc: Mel Gorman <mgorman@xxxxxxx>
> >> Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> >> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >> Cc: linux-kernel@xxxxxxxxxxxxxxx
> >> ---
> >>  kernel/sched/features.h | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> >> index 7481cd96f391..68d369cba9e4 100644
> >> --- a/kernel/sched/features.h
> >> +++ b/kernel/sched/features.h
> >> @@ -77,7 +77,7 @@ SCHED_FEAT(WARN_DOUBLE_CLOCK, false)
> >>  SCHED_FEAT(RT_PUSH_IPI, true)
> >>  #endif
> >>
> >> -SCHED_FEAT(RT_RUNTIME_SHARE, true)
> >> +SCHED_FEAT(RT_RUNTIME_SHARE, false)
> >>  SCHED_FEAT(LB_MIN, false)
> >>  SCHED_FEAT(ATTACH_AGE_LOAD, true)
> >>
> >> --
> >> 2.26.2
> >>
> >
> > Tested on an Android device and can no longer see long running RT
> > tasks (yes, Android have those for reasons).
> >
>
> So:
>
> Tested-by: Wei Wang <wvw@xxxxxxxxxx>
>
> ?

Yup, thanks for pushing this.

>
> Thanks!
> -- Daniel
>