Re: [PATCH] sched/fair: fix unthrottle_cfs_rq for leaf_cfs_rq list

[Phil Auld <pauld@redhat.com>]

On Mon, May 11, 2020 at 09:13:20PM +0200 Vincent Guittot wrote:
> Although not exactly identical, unthrottle_cfs_rq() and enqueue_task_fair()
> are quite close and follow the same sequence for enqueuing an entity in the
> cfs hierarchy. Modify unthrottle_cfs_rq() to use the same pattern as
> enqueue_task_fair(). This fixes a problem already faced with the latter and
> add an optimization in the last for_each_sched_entity loop.
> 
> Fixes: fe61468b2cb (sched/fair: Fix enqueue_task_fair warning)
> Reported-by Tao Zhou <zohooouoto@zoho.com.cn>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
> 
> This path applies on top of 20200507203612.GF19331@lorien.usersys.redhat.com
> and fixes similar problem for unthrottle_cfs_rq()
> 
>  kernel/sched/fair.c | 37 ++++++++++++++++++++++++++++---------
>  1 file changed, 28 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e2450c2e0747..4b73518aa25c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4803,26 +4803,44 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
>  	idle_task_delta = cfs_rq->idle_h_nr_running;
>  	for_each_sched_entity(se) {
>  		if (se->on_rq)
> -			enqueue = 0;
> +			break;
> +		cfs_rq = cfs_rq_of(se);
> +		enqueue_entity(cfs_rq, se, ENQUEUE_WAKEUP);
>  
> +		cfs_rq->h_nr_running += task_delta;
> +		cfs_rq->idle_h_nr_running += idle_task_delta;
> +
> +		/* end evaluation on encountering a throttled cfs_rq */
> +		if (cfs_rq_throttled(cfs_rq))
> +			goto unthrottle_throttle;
> +	}
> +
> +	for_each_sched_entity(se) {
>  		cfs_rq = cfs_rq_of(se);
> -		if (enqueue) {
> -			enqueue_entity(cfs_rq, se, ENQUEUE_WAKEUP);
> -		} else {
> -			update_load_avg(cfs_rq, se, 0);
> -			se_update_runnable(se);
> -		}
> +
> +		update_load_avg(cfs_rq, se, UPDATE_TG);
> +		se_update_runnable(se);
>  
>  		cfs_rq->h_nr_running += task_delta;
>  		cfs_rq->idle_h_nr_running += idle_task_delta;
>  
> +
> +		/* end evaluation on encountering a throttled cfs_rq */
>  		if (cfs_rq_throttled(cfs_rq))
> -			break;
> +			goto unthrottle_throttle;
> +
> +		/*
> +		 * One parent has been throttled and cfs_rq removed from the
> +		 * list. Add it back to not break the leaf list.
> +		 */
> +		if (throttled_hierarchy(cfs_rq))
> +			list_add_leaf_cfs_rq(cfs_rq);
>  	}
>  
>  	if (!se)
>  		add_nr_running(rq, task_delta);
>  
> +unthrottle_throttle:
>  	/*
>  	 * The cfs_rq_throttled() breaks in the above iteration can result in
>  	 * incomplete leaf list maintenance, resulting in triggering the
> @@ -4831,7 +4849,8 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
>  	for_each_sched_entity(se) {
>  		cfs_rq = cfs_rq_of(se);
>  
> -		list_add_leaf_cfs_rq(cfs_rq);
> +		if (list_add_leaf_cfs_rq(cfs_rq))
> +			break;
>  	}
>  
>  	assert_list_leaf_cfs_rq(rq);
> -- 
> 2.17.1
> 

I ran my reproducer test with this one as well. As expected, since
the first patch fixed the issue I was seeing and I wasn't hitting
the assert here anyway, I didn't hit the assert.

But I also didn't hit any other issues, new or old. 

It makes sense to use the same logic flow here as enqueue_task_fair.

Reviewed-by: Phil Auld <pauld@redhat.com>


Cheers,
Phil
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