Re: [PATCH V2] sched/fair: Fix that tasks are not constrained by cfs_b->quota on hotplug core, when hotplug core is offline and then online.

[Peter Zijlstra]

You forgot to Cc Ben, who gave you feedback on v1, which is rather poor
style. Also, I don't see how kernel-janitors is relevant to this patch.
This is very much not a janitorial thing.

(also, why send it twice?)

On Tue, Aug 30, 2016 at 10:12:40PM +0900, Jeehong Kim wrote:
> In case that CONFIG_HOTPLUG_CPU and CONFIG_CFS_BANDWIDTH is turned on
> and tasks in bandwidth controlled task group run on hotplug core,
> the tasks are not controlled by cfs_b->quota when hotplug core is offline
> and then online. The remaining tasks in task group consume all of
> cfs_b->quota on other cores.
> 
> The cause of this problem is described as below:
> 
> 1. When hotplug core is offline while tasks in task group run
> on hotplug core, unregister_fair_sched_group() deletes
> leaf_cfs_rq_list of tg->cfs_rq[cpu] from &rq_of(cfs_rq)->leaf_cfs_rq_list.
> 
> 2. Then, when hotplug core is online, update_runtime_enabled()
> registers cfs_b->quota on cfs_rq->runtime_enabled of all leaf cfs_rq
> on runqueue. However, because this is before enqueue_entity() adds
> &cfs_rq->leaf_cfs_rq_list on &rq_of(cfs_rq)->leaf_cfs_rq_list,
> cfs->quota is not register on cfs_rq->runtime_enabled.
> 
> To resolve this problem, this patch makes update_runtime_enabled()
> registers cfs_b->quota by using walk_tg_tree_from().


> +static int __maybe_unused __update_runtime_enabled(struct task_group *tg, void *data)
>  {
> +	struct rq *rq = data;
> +	struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
> +	struct cfs_bandwidth *cfs_b = &cfs_rq->tg->cfs_bandwidth;
> 
> +	raw_spin_lock(&cfs_b->lock);
> +	cfs_rq->runtime_enabled = cfs_b->quota != RUNTIME_INF;
> +	raw_spin_unlock(&cfs_b->lock);
> 
> +	return 0;
> +}
> +
> +static void __maybe_unused update_runtime_enabled(struct rq *rq)
> +{
> +	struct cfs_rq *cfs_rq = &rq->cfs;
> +
> +	/* register cfs_b->quota on the whole tg tree */
> +	rcu_read_lock();
> +	walk_tg_tree_from(cfs_rq->tg, __update_runtime_enabled, tg_nop, (void *)rq);
> +	rcu_read_unlock();
>  }

Looks ok, performance on hotplug doesn't really matter. Ben, you happy
with this?