Re: [PATCH 02/16] sched/fair: Consistent use of prev_cpu in wakeup path

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 23, 2016 at 11:58:44AM +0100, Morten Rasmussen wrote:
> In commit ac66f5477239 ("sched/numa: Introduce migrate_swap()")
> select_task_rq() got a 'cpu' argument to enable overriding of prev_cpu
> in special cases (NUMA task swapping). However, the
> select_task_rq_fair() helper functions: wake_affine() and
> select_idle_sibling(), still use task_cpu(p) directly to work out
> prev_cpu which leads to inconsistencies.
> 
> This patch passes prev_cpu (potentially overridden by NUMA code) into
> the helper functions to ensure prev_cpu is indeed the same cpu
> everywhere in the wakeup path.

Rik, can you get this ran through the NUMA benchmarks?

> 
> cc: Ingo Molnar <mingo@redhat.com>
> cc: Peter Zijlstra <peterz@infradead.org>
> 
> Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
> ---
>  kernel/sched/fair.c | 24 +++++++++++++-----------
>  1 file changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 218f8e8..c49e25a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -656,7 +656,7 @@ static u64 sched_vslice(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  }
>  
>  #ifdef CONFIG_SMP
> -static int select_idle_sibling(struct task_struct *p, int cpu);
> +static int select_idle_sibling(struct task_struct *p, int prev_cpu, int cpu);
>  static unsigned long task_h_load(struct task_struct *p);
>  
>  /*
> @@ -1502,7 +1502,8 @@ static void task_numa_compare(struct task_numa_env *env,
>  	 * Call select_idle_sibling to maybe find a better one.
>  	 */
>  	if (!cur)
> -		env->dst_cpu = select_idle_sibling(env->p, env->dst_cpu);
> +		env->dst_cpu = select_idle_sibling(env->p, env->src_cpu,
> +						   env->dst_cpu);
>  
>  assign:
>  	assigned = true;
> @@ -5013,18 +5014,18 @@ static int wake_wide(struct task_struct *p)
>  	return 1;
>  }
>  
> -static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
> +static int wake_affine(struct sched_domain *sd, struct task_struct *p,
> +		       int prev_cpu, int sync)
>  {
>  	s64 this_load, load;
>  	s64 this_eff_load, prev_eff_load;
> -	int idx, this_cpu, prev_cpu;
> +	int idx, this_cpu;
>  	struct task_group *tg;
>  	unsigned long weight;
>  	int balanced;
>  
>  	idx	  = sd->wake_idx;
>  	this_cpu  = smp_processor_id();
> -	prev_cpu  = task_cpu(p);
>  	load	  = source_load(prev_cpu, idx);
>  	this_load = target_load(this_cpu, idx);
>  
> @@ -5189,11 +5190,10 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
>  /*
>   * Try and locate an idle CPU in the sched_domain.
>   */
> -static int select_idle_sibling(struct task_struct *p, int target)
> +static int select_idle_sibling(struct task_struct *p, int prev, int target)
>  {
>  	struct sched_domain *sd;
>  	struct sched_group *sg;
> -	int i = task_cpu(p);
>  
>  	if (idle_cpu(target))
>  		return target;
> @@ -5201,8 +5201,8 @@ static int select_idle_sibling(struct task_struct *p, int target)
>  	/*
>  	 * If the prevous cpu is cache affine and idle, don't be stupid.
>  	 */
> -	if (i != target && cpus_share_cache(i, target) && idle_cpu(i))
> -		return i;
> +	if (prev != target && cpus_share_cache(prev, target) && idle_cpu(prev))
> +		return prev;
>  
>  	/*
>  	 * Otherwise, iterate the domains and find an eligible idle cpu.
> @@ -5223,6 +5223,8 @@ static int select_idle_sibling(struct task_struct *p, int target)
>  	for_each_lower_domain(sd) {
>  		sg = sd->groups;
>  		do {
> +			int i;
> +
>  			if (!cpumask_intersects(sched_group_cpus(sg),
>  						tsk_cpus_allowed(p)))
>  				goto next;
> @@ -5331,13 +5333,13 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
>  
>  	if (affine_sd) {
>  		sd = NULL; /* Prefer wake_affine over balance flags */
> -		if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))
> +		if (cpu != prev_cpu && wake_affine(affine_sd, p, prev_cpu, sync))
>  			new_cpu = cpu;
>  	}
>  
>  	if (!sd) {
>  		if (sd_flag & SD_BALANCE_WAKE) /* XXX always ? */
> -			new_cpu = select_idle_sibling(p, new_cpu);
> +			new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
>  
>  	} else while (sd) {
>  		struct sched_group *group;
> -- 
> 1.9.1
>