Messages in this thread | | | Date | Thu, 9 Apr 2020 11:25:58 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH 4/4] sched/deadline: Implement fallback mechanism for !fit case |
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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?
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?
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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