Re: [PATCH v3 5/5] sched/deadline: Implement fallback mechanism for !fit case

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

On 20/05/20 15:42, 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.
> 
> Favor task_cpu(p) if it has max capacity of !fitting CPUs so that
> find_later_rq() can potentially still return it (most likely cache-hot)
> early.
> 
> Signed-off-by: Luca Abeni <luca.abeni@santannapisa.it>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> ---
>  kernel/sched/cpudeadline.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> index 8630f2a40a3f..8cb06c8c7eb1 100644
> --- a/kernel/sched/cpudeadline.c
> +++ b/kernel/sched/cpudeadline.c
> @@ -121,19 +121,31 @@ 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 = capacity_orig_of(cpu);
> +
> +				if (cap > max_cap ||
> +				    (cpu == task_cpu(p) && 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);
>  
> -- 

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