Re: [PATCH v3 4/5] sched/deadline: Make DL capacity-aware

[Juri Lelli <juri.lelli@redhat.com>]

On 20/05/20 15:42, Dietmar Eggemann wrote:
> From: Luca Abeni <luca.abeni@santannapisa.it>
> 
> The current SCHED_DEADLINE (DL) scheduler uses a global EDF scheduling
> algorithm w/o considering CPU capacity or task utilization.
> This works well on homogeneous systems where DL tasks are guaranteed
> to have a bounded tardiness but presents issues on heterogeneous
> systems.
> 
> A DL task can migrate to a CPU which does not have enough CPU capacity
> to correctly serve the task (e.g. a task w/ 70ms runtime and 100ms
> period on a CPU w/ 512 capacity).
> 
> Add the DL fitness function dl_task_fits_capacity() for DL admission
> control on heterogeneous systems. A task fits onto a CPU if:
> 
>     CPU original capacity / 1024 >= task runtime / task deadline
> 
> Use this function on heterogeneous systems to try to find a CPU which
> meets this criterion during task wakeup, push and offline migration.
> 
> On homogeneous systems the original behavior of the DL admission
> control should be retained.
> 
> Signed-off-by: Luca Abeni <luca.abeni@santannapisa.it>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> ---
>  kernel/sched/cpudeadline.c | 14 +++++++++++++-
>  kernel/sched/deadline.c    | 18 ++++++++++++++----
>  kernel/sched/sched.h       | 15 +++++++++++++++
>  3 files changed, 42 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> index 5cc4012572ec..8630f2a40a3f 100644
> --- a/kernel/sched/cpudeadline.c
> +++ b/kernel/sched/cpudeadline.c
> @@ -121,7 +121,19 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
>  
>  	if (later_mask &&
>  	    cpumask_and(later_mask, cp->free_cpus, p->cpus_ptr)) {
> -		return 1;
> +		int cpu;
> +
> +		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))
> +				cpumask_clear_cpu(cpu, later_mask);
> +		}
> +
> +		if (!cpumask_empty(later_mask))
> +			return 1;
>  	} else {
>  		int best_cpu = cpudl_maximum(cp);
>  
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index fa8566517715..f2e8f5a36707 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1643,6 +1643,7 @@ static int
>  select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags)
>  {
>  	struct task_struct *curr;
> +	bool select_rq;
>  	struct rq *rq;
>  
>  	if (sd_flag != SD_BALANCE_WAKE)
> @@ -1662,10 +1663,19 @@ select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags)
>  	 * other hand, if it has a shorter deadline, we
>  	 * try to make it stay here, it might be important.
>  	 */
> -	if (unlikely(dl_task(curr)) &&
> -	    (curr->nr_cpus_allowed < 2 ||
> -	     !dl_entity_preempt(&p->dl, &curr->dl)) &&
> -	    (p->nr_cpus_allowed > 1)) {
> +	select_rq = unlikely(dl_task(curr)) &&
> +		    (curr->nr_cpus_allowed < 2 ||
> +		     !dl_entity_preempt(&p->dl, &curr->dl)) &&
> +		    p->nr_cpus_allowed > 1;
> +
> +	/*
> +	 * Take the capacity of the CPU into account to
> +	 * ensure it fits the requirement of the task.
> +	 */
> +	if (static_branch_unlikely(&sched_asym_cpucapacity))
> +		select_rq |= !dl_task_fits_capacity(p, cpu);
> +
> +	if (select_rq) {
>  		int target = find_later_rq(p);
>  
>  		if (target != -1 &&
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 14cb6a97e2d2..6ebbb1f353c4 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -317,6 +317,21 @@ static inline bool __dl_overflow(struct dl_bw *dl_b, unsigned long cap,
>  	       cap_scale(dl_b->bw, cap) < dl_b->total_bw - old_bw + new_bw;
>  }
>  
> +/*
> + * Verify the fitness of task @p to run on @cpu taking into account the
> + * CPU original capacity and the runtime/deadline ratio of the task.
> + *
> + * The function will return true if the CPU original capacity of the
> + * @cpu scaled by SCHED_CAPACITY_SCALE >= runtime/deadline ratio of the
> + * task and false otherwise.
> + */
> +static inline bool dl_task_fits_capacity(struct task_struct *p, int cpu)
> +{
> +	unsigned long cap = arch_scale_cpu_capacity(cpu);
> +
> +	return cap_scale(p->dl.dl_deadline, cap) >= p->dl.dl_runtime;
> +}
> +
>  extern void init_dl_bw(struct dl_bw *dl_b);
>  extern int  sched_dl_global_validate(void);
>  extern void sched_dl_do_global(void);
> -- 

Acked-by: Juri Lelli <juri.lelli@redhat.com>