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

[luca abeni <luca.abeni@santannapisa.it>]

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
> >