Re: [PATCH] nohz/full, sched/rt: Fix missed tick-reenabling bug in dequeue_task_rt

Peter Zijlstra
2022-07-20T12:50:53Z

On Wed, Jul 13, 2022 at 07:18:42PM +0200, Nicolas Saenz Julienne wrote:
> On Tue, 2022-06-28 at 11:22 +0200, Nicolas Saenz Julienne wrote:
> > dequeue_task_rt() only decrements 'rt_rq->rt_nr_running' after having
> > called sched_update_tick_dependency() preventing it from re-enabling the
> > tick on systems that no longer have pending SCHED_RT tasks but have
> > multiple runnable SCHED_OTHER tasks:
> >
> > dequeue_task_rt()
> > dequeue_rt_entity()
> > dequeue_rt_stack()
> > dequeue_top_rt_rq()
> > sub_nr_running() // decrements rq->nr_running
> > sched_update_tick_dependency()
> > sched_can_stop_tick() // checks rq->rt.rt_nr_running,
> > ...
> > __dequeue_rt_entity()
> > dec_rt_tasks() // decrements rq->rt.rt_nr_running
> > ...
> >
> > Every other scheduler class performs the operation in the opposite
> > order, and sched_update_tick_dependency() expects the values to be
> > updated as such. So avoid the misbehaviour by inverting the order in
> > which the above operations are performed in the RT scheduler.
> >
> > Fixes: 76d92ac305f2 ("sched: Migrate sched to use new tick dependency mask model")
> > Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
> > ---
>
> Ping :)

Thanks! got stuck in the retbleed backlog :/