Re: [PATCH v2 6/6] sched/deadline: Implement fallback mechanism for !fit case

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

Hi,

On 27/04/20 10:37, 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>
> ---
>  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..b6c7a0bc0880 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 = capacity_orig_of(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);

Think we touched upon this during v1 review, but I'm (still?) wondering
if we can do a little better, still considering only free cpus.

Can't we get into a situation that some of the (once free) big cpus have
been occupied by small tasks and now a big task enters the system and it
only finds small cpus available, were it could have fit into bigs if
small tasks were put onto small cpus?

I.e., shouldn't we always try to best fit among free cpus?

Thanks,

Juri