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