Re: [PATCH 4/4] sched/deadline: Implement fallback mechanism for !fit case

luca abeni
2020-04-09T13:00:28Z

Hi,

On Thu, 9 Apr 2020 11:25:58 +0100
Qais Yousef <qais.yousef@arm.com> wrote:

> On 04/08/20 11:50, Dietmar Eggemann wrote:
> > From: Luca Abeni <luca.abeni@santannapisa.it>
> >
> > When a task has a runtime that cannot be served within the
> > scheduling deadline by any of the idle CPU (later_mask) the task is
> > doomed to miss its deadline.
> >
> > This can happen since the SCHED_DEADLINE admission control
> > guarantees only bounded tardiness and not the hard respect of all
> > deadlines. In this case try to select the idle CPU with the largest
> > CPU capacity to minimize tardiness.
> >
> > Signed-off-by: Luca Abeni <luca.abeni@santannapisa.it>
> > Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > ---
>
> Outside of the scope of this series. But does it make sense to make
> sched_setattr() fail to create a new deadline task if the system will
> be overcommitted, hence causing some dl tasks to miss their deadlines?

The problem is that with multiple processors/cores it is not easy to
know in advance if any task will miss a deadline (see section 3.3 of
Documentation/scheduler/sched-deadline.rst).

The admission control we are currently using should prevent
SCHED_DEADLINE tasks from overloading the system (starving non-deadline
tasks); proving hard deadline guarantees with global EDF scheduling is
much more difficult (and could be probably done in user-space, I think).

> If some overcommitting is fine (some deadlines are soft and are okay
> to fail every once in a while), does it make sense for this to be a
> tunable of how much the system can be overcommitted before
> disallowing new DL tasks to be created?

There is already a tunable for the SCHED_DEADLINE admission test
(/proc/sys/kernel/sched_rt_{runtime,period}_us, if I understand well
what you are suggesting). The problem is that it is not easy to find a
value for this tunable that guarantees the hard respect of all
deadlines.

But IMHO if someone really wants hard deadline guarantees it is better
to use partitioned scheduling (see Section 5 of the SCHED_DEADLINE
documentation).

Luca

>
> Just thinking out loudly. This fallback is fine, but it made me think
> why did we have to end up in a situation that we can fail in the
> first place since the same info is available when a new DL task is
> created, and being preventative might be a better approach..
>
> Thanks
>
> --
> Qais Yousef
>
> > kernel/sched/cpudeadline.c | 19 +++++++++++++++----
> > 1 file changed, 15 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> > index 8630f2a40a3f..8525d73e3de4 100644
> > --- a/kernel/sched/cpudeadline.c
> > +++ b/kernel/sched/cpudeadline.c
> > @@ -121,19 +121,30 @@ int cpudl_find(struct cpudl *cp, struct
> > task_struct *p,
> > if (later_mask &&
> > cpumask_and(later_mask, cp->free_cpus, p->cpus_ptr)) {
> > - int cpu;
> > + unsigned long cap, max_cap = 0;
> > + int cpu, max_cpu = -1;
> >
> > if
> > (!static_branch_unlikely(&sched_asym_cpucapacity)) return 1;
> >
> > /* Ensure the capacity of the CPUs fits the task.
> > */ for_each_cpu(cpu, later_mask) {
> > - if (!dl_task_fits_capacity(p, cpu))
> > + if (!dl_task_fits_capacity(p, cpu)) {
> > cpumask_clear_cpu(cpu, later_mask);
> > +
> > + cap = arch_scale_cpu_capacity(cpu);
> > +
> > + if (cap > max_cap) {
> > + max_cap = cap;
> > + max_cpu = cpu;
> > + }
> > + }
> > }
> >
> > - if (!cpumask_empty(later_mask))
> > - return 1;
> > + if (cpumask_empty(later_mask))
> > + cpumask_set_cpu(max_cpu, later_mask);
> > +
> > + return 1;
> > } else {
> > int best_cpu = cpudl_maximum(cp);
> >
> > --
> > 2.17.1
> >