Re: [PATCH 4/5] sched/fair: Move the throttled tasks to a local list in tg_unthrottle_up()

[Benjamin Segall]

K Prateek Nayak <kprateek.nayak@xxxxxxx> writes:

> An update_curr() during the enqueue of throttled task will start
> throttling the hierarchy from subsequent commit. This can lead to
> tg_throttle_down() seeing non-empty throttled_limbo_list for the cfs_rq
> attaching the task from throttled_limbo_list one by one. For example:
>
>      R
>      |
>      A
>     / \
>   *B   C
>        |
>        rq->curr
>
> *B is throttled with tasks on hte limbo list. When the tasks are
> unthrottled via tg_unthrottle_up() and entity of group B is placed onto
> A, update_curr() is called to catch up the vruntime and it may throttle
> group A causing the subsequent tg_throttle_down() to see the pending
> task's on B's limbo list.
>
>   tg_unthrottle_up()
>     /* --cfs_rq->throttle_count == 0 */
>     list_for_each_entry_safe(p, cfs_rq->throttled_limbo_list)
>       enqueue_task_fair()
>         enqueue_entity(se /* B->se */)
>           update_curr(cfs_rq /* A->gcfs_rq */)
>             account_cfs_rq_runtime(cfs_rq)
>               throttle_cfs_rq(cfs_rq /* A->gcfs_rq */ )
>                 tg_throttle_down()
>                   /* Reaches B->cfs_rq with throttle_count == 0 */
>
>                   !!! !list_empty(&cfs_rq->throttled_limbo_list)) !!!
>
> Move the tasks from throttled_limbo_list onto a local list before
> starting the unthrottle to prevent the splat described above. If the
> hierarchy is throttled again in middle of an unthrottle, put the pending
> tasks back onto the limbo list to prevent running them unnecessarily.

And for extra fun, in order to get here we need to have just finished
throttle_cfs_rq_work and its resched_curr, but not have managed to reach
schedule yet when the unthrottle hits. But yeah, that can happen.

Reviewed-By: Benjamin Segall <bsegall@xxxxxxxxxx>


>
> Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> ---
>  kernel/sched/fair.c | 23 +++++++++++++++++++++--
>  1 file changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b3b3172702a9..c48eaf2d7919 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6710,6 +6710,7 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
>  	struct rq *rq = data;
>  	struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
>  	struct task_struct *p, *tmp;
> +	LIST_HEAD(throttled_tasks);
>  
>  	/*
>  	 * If cfs_rq->curr is set, the cfs_rq might not have caught up
> @@ -6740,13 +6741,31 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
>  		cfs_rq->throttled_clock_self_time += delta;
>  	}
>  
> +	/*
> +	 * Move the tasks to a local list since an update_curr() during
> +	 * enqueue_task_fair() can throttle a higher cfs_rq, and it can
> +	 * see the "throttled_limbo_list" being non-empty in
> +	 * tg_throttle_down() if throttle_count turned 0 above.
> +	 */
> +	list_splice_init(&cfs_rq->throttled_limbo_list, &throttled_tasks);
> +
>  	/* Re-enqueue the tasks that have been throttled at this level. */
> -	list_for_each_entry_safe(p, tmp, &cfs_rq->throttled_limbo_list, throttle_node) {
> +	list_for_each_entry_safe(p, tmp, &throttled_tasks, throttle_node) {
> +		/*
> +		 * Back to being throttled! Break out and put the remaining
> +		 * tasks back onto the limbo_list to prevent running them
> +		 * unnecessarily.
> +		 */
> +		if (cfs_rq->throttle_count)
> +			break;
> +
>  		list_del_init(&p->throttle_node);
>  		p->throttled = false;
> -		enqueue_task_fair(rq_of(cfs_rq), p, ENQUEUE_WAKEUP);
> +		enqueue_task_fair(rq, p, ENQUEUE_WAKEUP);
>  	}
>  
> +	list_splice(&throttled_tasks, &cfs_rq->throttled_limbo_list);
> +
>  	/* Add cfs_rq with load or one or more already running entities to the list */
>  	if (!cfs_rq_is_decayed(cfs_rq))
>  		list_add_leaf_cfs_rq(cfs_rq);