Re: [PATCH] sched/fair: Ignore percpu threads for imbalance pulls

[Valentin Schneider <valentin.schneider@arm.com>]

On 17/02/21 17:38, Lingutla Chandrasekhar wrote:
> In load balancing, when balancing group is unable to pull task
> due to ->cpus_ptr constraints from busy group, then it sets
> LBF_SOME_PINNED to lb env flags, as a consequence, sgc->imbalance
> is set for its parent domain level. which makes the group
> classified as imbalance to get help from another balancing cpu.
>
> Consider a 4-CPU big.LITTLE system with CPUs 0-1 as LITTLEs and
> CPUs 2-3 as Bigs with below scenario:
> - CPU0 doing newly_idle balancing
> - CPU1 running percpu kworker and RT task (small tasks)
> - CPU2 running 2 big tasks
> - CPU3 running 1 medium task
>
> While CPU0 is doing newly_idle load balance at MC level, it fails to
> pull percpu kworker from CPU1 and sets LBF_SOME_PINNED to lb env flag
> and set sgc->imbalance at DIE level domain. As LBF_ALL_PINNED not cleared,
> it tries to redo the balancing by clearing CPU1 in env cpus, but it don't
> find other busiest_group, so CPU0 stops balacing at MC level without
> clearing 'sgc->imbalance' and restart the load balacing at DIE level.
>
> And CPU0 (balancing cpu) finds LITTLE's group as busiest_group with group
> type as imbalance, and Bigs that classified the level below imbalance type
> would be ignored to pick as busiest, and the balancing would be aborted
> without pulling any tasks (by the time, CPU1 might not have running tasks).
>
> It is suboptimal decision to classify the group as imbalance due to
> percpu threads. So don't use LBF_SOME_PINNED for per cpu threads.
>

Sounds like you've stumbled on the same thing I'm trying to fix in

  http://lore.kernel.org/r/20210128183141.28097-8-valentin.schneider@arm.com

(I'm currently working on a v2)

Now, I'd tend to agree that if we could prevent pcpu kworkers from
interfering with load-balance altogether, that would indeed be much
better than trying to deal with the group_imbalanced faff further down the
line (which is what I've been doing).

> Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 04a3ce20da67..44a05ad8c96b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7560,7 +7560,9 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>  
>  		schedstat_inc(p->se.statistics.nr_failed_migrations_affine);
>  
> -		env->flags |= LBF_SOME_PINNED;
> +		/* Ignore percpu threads for imbalance pulls. */
> +		if (p->nr_cpus_allowed > 1)
> +			env->flags |= LBF_SOME_PINNED;
>  
>  		/*
>  		 * Remember if this task can be migrated to any other CPU in

Unlike user tasks, pcpu kworkers have a stable affinity (with some hotplug
quirks), so perhaps we could do this instead:

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8a8bd7b13634..84fca350b9ae 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7539,6 +7539,9 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 	if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
 		return 0;
 
+	if (kthread_is_per_cpu(p))
+		return 0;
+
 	if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) {
 		int cpu;
 
---

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