Re: [PATCH v2] sched: rt: Make RT capacity aware

[Vincent Guittot]

On Wed, 9 Oct 2019 at 12:46, Qais Yousef <qais.yousef@xxxxxxx> wrote:
>
> Capacity Awareness refers to the fact that on heterogeneous systems
> (like Arm big.LITTLE), the capacity of the CPUs is not uniform, hence
> when placing tasks we need to be aware of this difference of CPU
> capacities.
>
> In such scenarios we want to ensure that the selected CPU has enough
> capacity to meet the requirement of the running task. Enough capacity
> means here that capacity_orig_of(cpu) >= task.requirement.
>
> The definition of task.requirement is dependent on the scheduling class.
>
> For CFS, utilization is used to select a CPU that has >= capacity value
> than the cfs_task.util.
>
>         capacity_orig_of(cpu) >= cfs_task.util
>
> DL isn't capacity aware at the moment but can make use of the bandwidth
> reservation to implement that in a similar manner CFS uses utilization.
> The following patchset implements that:
>
> https://lore.kernel.org/lkml/20190506044836.2914-1-luca.abeni@xxxxxxxxxxxxxxx/
>
>         capacity_orig_of(cpu)/SCHED_CAPACITY >= dl_deadline/dl_runtime
>
> For RT we don't have a per task utilization signal and we lack any
> information in general about what performance requirement the RT task
> needs. But with the introduction of uclamp, RT tasks can now control
> that by setting uclamp_min to guarantee a minimum performance point.
>
> ATM the uclamp value are only used for frequency selection; but on
> heterogeneous systems this is not enough and we need to ensure that the
> capacity of the CPU is >= uclamp_min. Which is what implemented here.
>
>         capacity_orig_of(cpu) >= rt_task.uclamp_min
>
> Note that by default uclamp.min is 1024, which means that RT tasks will
> always be biased towards the big CPUs, which make for a better more
> predictable behavior for the default case.

hmm... big cores are not always the best choices for rt tasks, they
generally took more time to wake up or to switch context because of
the pipeline depth and others branch predictions

>
> Must stress that the bias acts as a hint rather than a definite
> placement strategy. For example, if all big cores are busy executing
> other RT tasks we can't guarantee that a new RT task will be placed
> there.
>
> On non-heterogeneous systems the original behavior of RT should be
> retained. Similarly if uclamp is not selected in the config.
>
> Signed-off-by: Qais Yousef <qais.yousef@xxxxxxx>
> ---
>
> Changes in v2:
>         - Use cpupri_find() to check the fitness of the task instead of
>           sprinkling find_lowest_rq() with several checks of
>           rt_task_fits_capacity().
>
>           The selected implementation opted to pass the fitness function as an
>           argument rather than call rt_task_fits_capacity() capacity which is
>           a cleaner to keep the logical separation of the 2 modules; but it
>           means the compiler has less room to optimize rt_task_fits_capacity()
>           out when it's a constant value.
>
> The logic is not perfect. For example if a 'small' task is occupying a big CPU
> and another big task wakes up; we won't force migrate the small task to clear
> the big cpu for the big task that woke up.
>
> IOW, the logic is best effort and can't give hard guarantees. But improves the
> current situation where a task can randomly end up on any CPU regardless of
> what it needs. ie: without this patch an RT task can wake up on a big or small
> CPU, but with this it will always wake up on a big CPU (assuming the big CPUs
> aren't overloaded) - hence provide a consistent performance.
>
> I'm looking at ways to improve this best effort, but this patch should be
> a good start to discuss our Capacity Awareness requirement. There's a trade-off
> of complexity to be made here and I'd like to keep things as simple as
> possible and build on top as needed.
>
>
>  kernel/sched/cpupri.c | 23 ++++++++++--
>  kernel/sched/cpupri.h |  4 ++-
>  kernel/sched/rt.c     | 81 +++++++++++++++++++++++++++++++++++--------
>  3 files changed, 91 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c
> index b7abca987d94..799791c01d60 100644
> --- a/kernel/sched/cpupri.c
> +++ b/kernel/sched/cpupri.c
> @@ -57,7 +57,8 @@ static int convert_prio(int prio)
>   * Return: (int)bool - CPUs were found
>   */
>  int cpupri_find(struct cpupri *cp, struct task_struct *p,
> -               struct cpumask *lowest_mask)
> +               struct cpumask *lowest_mask,
> +               bool (*fitness_fn)(struct task_struct *p, int cpu))
>  {
>         int idx = 0;
>         int task_pri = convert_prio(p->prio);
> @@ -98,6 +99,8 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
>                         continue;
>
>                 if (lowest_mask) {
> +                       int cpu;
> +
>                         cpumask_and(lowest_mask, p->cpus_ptr, vec->mask);
>
>                         /*
> @@ -108,7 +111,23 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
>                          * condition, simply act as though we never hit this
>                          * priority level and continue on.
>                          */
> -                       if (cpumask_any(lowest_mask) >= nr_cpu_ids)
> +                       if (cpumask_empty(lowest_mask))
> +                               continue;
> +
> +                       if (!fitness_fn)
> +                               return 1;
> +
> +                       /* Ensure the capacity of the CPUs fit the task */
> +                       for_each_cpu(cpu, lowest_mask) {
> +                               if (!fitness_fn(p, cpu))
> +                                       cpumask_clear_cpu(cpu, lowest_mask);
> +                       }
> +
> +                       /*
> +                        * If no CPU at the current priority can fit the task
> +                        * continue looking
> +                        */
> +                       if (cpumask_empty(lowest_mask))
>                                 continue;
>                 }
>
> diff --git a/kernel/sched/cpupri.h b/kernel/sched/cpupri.h
> index 7dc20a3232e7..32dd520db11f 100644
> --- a/kernel/sched/cpupri.h
> +++ b/kernel/sched/cpupri.h
> @@ -18,7 +18,9 @@ struct cpupri {
>  };
>
>  #ifdef CONFIG_SMP
> -int  cpupri_find(struct cpupri *cp, struct task_struct *p, struct cpumask *lowest_mask);
> +int  cpupri_find(struct cpupri *cp, struct task_struct *p,
> +                struct cpumask *lowest_mask,
> +                bool (*fitness_fn)(struct task_struct *p, int cpu));
>  void cpupri_set(struct cpupri *cp, int cpu, int pri);
>  int  cpupri_init(struct cpupri *cp);
>  void cpupri_cleanup(struct cpupri *cp);
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index ebaa4e619684..3a68054e15b3 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -437,6 +437,45 @@ static inline int on_rt_rq(struct sched_rt_entity *rt_se)
>         return rt_se->on_rq;
>  }
>
> +#ifdef CONFIG_UCLAMP_TASK
> +/*
> + * Verify the fitness of task @p to run on @cpu taking into account the uclamp
> + * settings.
> + *
> + * This check is only important for heterogeneous systems where uclamp_min value
> + * is higher than the capacity of a @cpu. For non-heterogeneous system this
> + * function will always return true.
> + *
> + * The function will return true if the capacity of the @cpu is >= the
> + * uclamp_min and false otherwise.
> + *
> + * Note that uclamp_min will be clamped to uclamp_max if uclamp_min
> + * > uclamp_max.
> + */
> +inline bool rt_task_fits_capacity(struct task_struct *p, int cpu)
> +{
> +       unsigned int min_cap;
> +       unsigned int max_cap;
> +       unsigned int cpu_cap;
> +
> +       /* Only heterogeneous systems can benefit from this check */
> +       if (!static_branch_unlikely(&sched_asym_cpucapacity))
> +               return true;
> +
> +       min_cap = uclamp_eff_value(p, UCLAMP_MIN);
> +       max_cap = uclamp_eff_value(p, UCLAMP_MAX);
> +
> +       cpu_cap = capacity_orig_of(cpu);
> +
> +       return cpu_cap >= min(min_cap, max_cap);
> +}
> +#else
> +static inline bool rt_task_fits_capacity(struct task_struct *p, int cpu)
> +{
> +       return true;
> +}
> +#endif
> +
>  #ifdef CONFIG_RT_GROUP_SCHED
>
>  static inline u64 sched_rt_runtime(struct rt_rq *rt_rq)
> @@ -1391,6 +1430,7 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
>  {
>         struct task_struct *curr;
>         struct rq *rq;
> +       bool test;
>
>         /* For anything but wake ups, just return the task_cpu */
>         if (sd_flag != SD_BALANCE_WAKE && sd_flag != SD_BALANCE_FORK)
> @@ -1422,10 +1462,16 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
>          *
>          * This test is optimistic, if we get it wrong the load-balancer
>          * will have to sort it out.
> +        *
> +        * We take into account the capacity of the cpu to ensure it fits the
> +        * requirement of the task - which is only important on heterogeneous
> +        * systems like big.LITTLE.
>          */
> -       if (curr && unlikely(rt_task(curr)) &&
> -           (curr->nr_cpus_allowed < 2 ||
> -            curr->prio <= p->prio)) {
> +       test = curr &&
> +              unlikely(rt_task(curr)) &&
> +              (curr->nr_cpus_allowed < 2 || curr->prio <= p->prio);
> +
> +       if (test || !rt_task_fits_capacity(p, cpu)) {
>                 int target = find_lowest_rq(p);
>
>                 /*
> @@ -1449,7 +1495,7 @@ static void check_preempt_equal_prio(struct rq *rq, struct task_struct *p)
>          * let's hope p can move out.
>          */
>         if (rq->curr->nr_cpus_allowed == 1 ||
> -           !cpupri_find(&rq->rd->cpupri, rq->curr, NULL))
> +           !cpupri_find(&rq->rd->cpupri, rq->curr, NULL, NULL))
>                 return;
>
>         /*
> @@ -1457,7 +1503,7 @@ static void check_preempt_equal_prio(struct rq *rq, struct task_struct *p)
>          * see if it is pushed or pulled somewhere else.
>          */
>         if (p->nr_cpus_allowed != 1
> -           && cpupri_find(&rq->rd->cpupri, p, NULL))
> +           && cpupri_find(&rq->rd->cpupri, p, NULL, NULL))
>                 return;
>
>         /*
> @@ -1600,7 +1646,8 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p, struct rq_fla
>  static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
>  {
>         if (!task_running(rq, p) &&
> -           cpumask_test_cpu(cpu, p->cpus_ptr))
> +           cpumask_test_cpu(cpu, p->cpus_ptr) &&
> +           rt_task_fits_capacity(p, cpu))
>                 return 1;
>
>         return 0;
> @@ -1642,7 +1689,8 @@ static int find_lowest_rq(struct task_struct *task)
>         if (task->nr_cpus_allowed == 1)
>                 return -1; /* No other targets possible */
>
> -       if (!cpupri_find(&task_rq(task)->rd->cpupri, task, lowest_mask))
> +       if (!cpupri_find(&task_rq(task)->rd->cpupri, task, lowest_mask,
> +                        rt_task_fits_capacity))
>                 return -1; /* No targets found */
>
>         /*
> @@ -2146,12 +2194,14 @@ static void pull_rt_task(struct rq *this_rq)
>   */
>  static void task_woken_rt(struct rq *rq, struct task_struct *p)
>  {
> -       if (!task_running(rq, p) &&
> -           !test_tsk_need_resched(rq->curr) &&
> -           p->nr_cpus_allowed > 1 &&
> -           (dl_task(rq->curr) || rt_task(rq->curr)) &&
> -           (rq->curr->nr_cpus_allowed < 2 ||
> -            rq->curr->prio <= p->prio))
> +       bool need_to_push = !task_running(rq, p) &&
> +                           !test_tsk_need_resched(rq->curr) &&
> +                           p->nr_cpus_allowed > 1 &&
> +                           (dl_task(rq->curr) || rt_task(rq->curr)) &&
> +                           (rq->curr->nr_cpus_allowed < 2 ||
> +                            rq->curr->prio <= p->prio);
> +
> +       if (need_to_push || !rt_task_fits_capacity(p, cpu_of(rq)))
>                 push_rt_tasks(rq);
>  }
>
> @@ -2223,7 +2273,10 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
>          */
>         if (task_on_rq_queued(p) && rq->curr != p) {
>  #ifdef CONFIG_SMP
> -               if (p->nr_cpus_allowed > 1 && rq->rt.overloaded)
> +               bool need_to_push = rq->rt.overloaded ||
> +                                   !rt_task_fits_capacity(p, cpu_of(rq));
> +
> +               if (p->nr_cpus_allowed > 1 && need_to_push)
>                         rt_queue_push_tasks(rq);
>  #endif /* CONFIG_SMP */
>                 if (p->prio < rq->curr->prio && cpu_online(cpu_of(rq)))
> --
> 2.17.1
>