Messages in this thread Patch in this message | | | Date | Thu, 9 Apr 2020 15:00:10 +0200 | From | luca abeni <> | Subject | Re: [PATCH 4/4] sched/deadline: Implement fallback mechanism for !fit case |
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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 > >
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