Re: [PATCH v3 1/7] sched/fair: Ignore percpu threads for imbalance pulls

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 11/03/2021 13:05, Valentin Schneider wrote:
> From: Lingutla Chandrasekhar <clingutla@codeaurora.org>
> 
> 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

Does it have to be a big.LITTLE system? I assume this issue also happens
on an SMP system.

> CPUs 2-3 as Bigs with below scenario:
> - CPU0 doing newly_idle balancing
> - CPU1 running percpu kworker and RT task (small tasks)

What's the role of the small RT task here in the story?

> - 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.

This sentence mentioned per-cpu threads (and so does the patch name) but
the implementation (only) deals with per-cpu kernel threads. IMHO, it
would be good to align this.

> 
> Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>
> [Use kthread_is_per_cpu() rather than p->nr_cpus_allowed]
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
>  kernel/sched/fair.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2e2ab1e00ef9..83aea97fbf22 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7565,6 +7565,10 @@ 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;
>  
> +	/* Disregard pcpu kthreads; they are where they need to be. */
> +	if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))
> +		return 0;
> +
>  	if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) {
>  		int cpu;
>  
>