Re: [PATCH] sched/fair: Consider cpu affinity when allowing NUMA imbalance in find_idlest_group

[Mel Gorman]

On Mon, Feb 07, 2022 at 09:29:21PM +0530, K Prateek Nayak wrote:
> Neither the sched/tip nor Mel's v5 patchset [1] provides an optimal
> new-task wakeup strategy when the tasks are affined to a subset of cpus
> which can result in piling of tasks on the same set of CPU in a NUMA
> group despite there being other cpus in a different NUMA group where the
> task could have run in. A good placement makes a difference especially
> in case of short lived task where the delay in load balancer kicking in
> can cause degradation in perfromance.
> 

Thanks.

V6 was posted based on previous feedback. While this patch is building
on top of it, please add Acked-by or Tested-by if the imbalance series
helps the general problem of handling imbalances when there are multiple
last level caches.

> <SNIP>
>
> Aggressive NUMA balancing is only done when needed. We select the
> minimum of number of allowed cpus in sched group and the calculated
> sd.imb_numa_nr as our imbalance threshold and the default behavior
> of mel-v5 is only modified when the former is smaller than
> the latter.
> 

In this context, it should be safe to reuse select_idle_mask like this
build tested patch

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 538756bd8e7f..1e759c21371b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9128,6 +9128,8 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
 
 	case group_has_spare:
 		if (sd->flags & SD_NUMA) {
+			struct cpumask *cpus;
+			int imb;
 #ifdef CONFIG_NUMA_BALANCING
 			int idlest_cpu;
 			/*
@@ -9145,10 +9147,15 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
 			 * Otherwise, keep the task close to the wakeup source
 			 * and improve locality if the number of running tasks
 			 * would remain below threshold where an imbalance is
-			 * allowed. If there is a real need of migration,
-			 * periodic load balance will take care of it.
+			 * allowed while accounting for the possibility the
+			 * task is pinned to a subset of CPUs.  If there is a
+			 * real need of migration, periodic load balance will
+			 * take care of it.
 			 */
-			if (allow_numa_imbalance(local_sgs.sum_nr_running + 1, sd->imb_numa_nr))
+			cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
+			cpumask_and(cpus, sched_group_span(local), p->cpus_ptr);
+			imb = min(cpumask_weight(cpus), sd->imb_numa_nr);
+			if (allow_numa_imbalance(local_sgs.sum_nr_running + 1, imb))
 				return NULL;
 		}