Re: [PATCH] sched/deadline: Make find_later_rq() choose a closer cpu in topology

[Byungchul Park]

On Tue, Mar 21, 2017 at 10:38:20AM +0900, Byungchul Park wrote:
> When cpudl_find() returns any among free_cpus, the cpu might not be
> closer than others, considering sched domain. For example:
> 
>    this_cpu: 15
>    free_cpus: 0, 1,..., 14 (== later_mask)
>    best_cpu: 0
> 
>    topology:
> 
>    0 --+
>        +--+
>    1 --+  |
>           +-- ... --+
>    2 --+  |         |
>        +--+         |
>    3 --+            |
> 
>    ...             ...
> 
>    12 --+           |
>         +--+        |
>    13 --+  |        |
>            +-- ... -+
>    14 --+  |
>         +--+
>    15 --+
> 
> In this case, it would be best to select 14 since it's a free cpu and
> closest to 15(this_cpu). However, currently the code select 0(best_cpu)
> even though that's just any among free_cpus. Fix it.

Hello,

I think it would be better to split this into two patches:

1. Clean up cpudl_find() at cpudeadline.c.
2. Make it choose a closer cpu in topology.

I will do it if you agree with my original purpose.

> 
> Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
> ---
>  kernel/sched/cpudeadline.c | 20 ++++++++------------
>  kernel/sched/deadline.c    | 29 +++++++++++++++--------------
>  2 files changed, 23 insertions(+), 26 deletions(-)
> 
> diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> index fba235c..cb36727 100644
> --- a/kernel/sched/cpudeadline.c
> +++ b/kernel/sched/cpudeadline.c
> @@ -119,7 +119,7 @@ static inline int cpudl_maximum(struct cpudl *cp)
>   * @p: the task
>   * @later_mask: a mask to fill in with the selected CPUs (or NULL)
>   *
> - * Returns: int - best CPU (heap maximum if suitable)
> + * Returns: (int)bool - CPUs were found
>   */
>  int cpudl_find(struct cpudl *cp, struct task_struct *p,
>  	       struct cpumask *later_mask)
> @@ -127,21 +127,17 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
>  	int best_cpu = -1;
>  	const struct sched_dl_entity *dl_se = &p->dl;
>  
> -	if (later_mask &&
> -	    cpumask_and(later_mask, cp->free_cpus, &p->cpus_allowed)) {
> -		best_cpu = cpumask_any(later_mask);
> -		goto out;
> -	} else if (cpumask_test_cpu(cpudl_maximum(cp), &p->cpus_allowed) &&
> -			dl_time_before(dl_se->deadline, cp->elements[0].dl)) {
> -		best_cpu = cpudl_maximum(cp);
> -		if (later_mask)
> +	if (later_mask) {
> +		if (!cpumask_and(later_mask, cp->free_cpus, &p->cpus_allowed) &&
> +		    cpumask_test_cpu(cpudl_maximum(cp), &p->cpus_allowed) &&
> +		    dl_time_before(dl_se->deadline, cp->elements[0].dl)) {
> +			best_cpu = cpudl_maximum(cp);
>  			cpumask_set_cpu(best_cpu, later_mask);
> +		}
>  	}
>  
> -out:
>  	WARN_ON(best_cpu != -1 && !cpu_present(best_cpu));
> -
> -	return best_cpu;
> +	return best_cpu != -1;
>  }
>  
>  /*
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index a2ce590..45cc3d3 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1324,7 +1324,7 @@ static int find_later_rq(struct task_struct *task)
>  	struct sched_domain *sd;
>  	struct cpumask *later_mask = this_cpu_cpumask_var_ptr(local_cpu_mask_dl);
>  	int this_cpu = smp_processor_id();
> -	int best_cpu, cpu = task_cpu(task);
> +	int cpu = task_cpu(task);
>  
>  	/* Make sure the mask is initialized first */
>  	if (unlikely(!later_mask))
> @@ -1337,17 +1337,14 @@ static int find_later_rq(struct task_struct *task)
>  	 * We have to consider system topology and task affinity
>  	 * first, then we can look for a suitable cpu.
>  	 */
> -	best_cpu = cpudl_find(&task_rq(task)->rd->cpudl,
> -			task, later_mask);
> -	if (best_cpu == -1)
> +	if (!cpudl_find(&task_rq(task)->rd->cpudl, task, later_mask))
>  		return -1;
>  
>  	/*
> -	 * If we are here, some target has been found,
> -	 * the most suitable of which is cached in best_cpu.
> -	 * This is, among the runqueues where the current tasks
> -	 * have later deadlines than the task's one, the rq
> -	 * with the latest possible one.
> +	 * If we are here, some targets have been found, including
> +	 * the most suitable which is, among the runqueues where the
> +	 * current tasks have later deadlines than the task's one, the
> +	 * rq with the latest possible one.
>  	 *
>  	 * Now we check how well this matches with task's
>  	 * affinity and system topology.
> @@ -1367,6 +1364,7 @@ static int find_later_rq(struct task_struct *task)
>  	rcu_read_lock();
>  	for_each_domain(cpu, sd) {
>  		if (sd->flags & SD_WAKE_AFFINE) {
> +			int closest_cpu;
>  
>  			/*
>  			 * If possible, preempting this_cpu is
> @@ -1378,14 +1376,17 @@ static int find_later_rq(struct task_struct *task)
>  				return this_cpu;
>  			}
>  
> +			closest_cpu = cpumask_first_and(later_mask,
> +							sched_domain_span(sd));
>  			/*
> -			 * Last chance: if best_cpu is valid and is
> -			 * in the mask, that becomes our choice.
> +			 * Last chance: if a cpu being in both later_mask
> +			 * and current sd span is valid, that becomes our
> +			 * choice. Of course, the latest possible cpu is
> +			 * already under consideration through later_mask.
>  			 */
> -			if (best_cpu < nr_cpu_ids &&
> -			    cpumask_test_cpu(best_cpu, sched_domain_span(sd))) {
> +			if (closest_cpu < nr_cpu_ids) {
>  				rcu_read_unlock();
> -				return best_cpu;
> +				return closest_cpu;
>  			}
>  		}
>  	}
> -- 
> 1.9.1