Commit fdaba61e authored by Rik van Riel's avatar Rik van Riel Committed by Ingo Molnar
Browse files

sched/fair: Ensure that the CFS parent is added after unthrottling 



Ensure that a CFS parent will be in the list whenever one of its children is also
in the list.

A warning on rq->tmp_alone_branch != &rq->leaf_cfs_rq_list has been
reported while running LTP test cfs_bandwidth01.

Odin Ugedal found the root cause:

	$ tree /sys/fs/cgroup/ltp/ -d --charset=ascii
	/sys/fs/cgroup/ltp/
	|-- drain
	`-- test-6851
	    `-- level2
		|-- level3a
		|   |-- worker1
		|   `-- worker2
		`-- level3b
		    `-- worker3

Timeline (ish):
- worker3 gets throttled
- level3b is decayed, since it has no more load
- level2 get throttled
- worker3 get unthrottled
- level2 get unthrottled
  - worker3 is added to list
  - level3b is not added to list, since nr_running==0 and is decayed

 [ Vincent Guittot: Rebased and updated to fix for the reported warning. ]

Fixes: a7b359fc ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
Reported-by: default avatarSachin Sant <sachinp@linux.vnet.ibm.com>
Suggested-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: default avatarRik van Riel <riel@surriel.com>
Signed-off-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Tested-by: default avatarSachin Sant <sachinp@linux.vnet.ibm.com>
Acked-by: default avatarOdin Ugedal <odin@uged.al>
Link: https://lore.kernel.org/r/20210621174330.11258-1-vincent.guittot@linaro.org
parent a96bfed6

kernel/sched/fair.c

+28 −0
Original line number Diff line number Diff line
@@ -3298,6 +3298,31 @@ static inline void cfs_rq_util_change(struct cfs_rq *cfs_rq, int flags) 


#ifdef CONFIG_SMP

#ifdef CONFIG_FAIR_GROUP_SCHED

/*

 * Because list_add_leaf_cfs_rq always places a child cfs_rq on the list

 * immediately before a parent cfs_rq, and cfs_rqs are removed from the list

 * bottom-up, we only have to test whether the cfs_rq before us on the list

 * is our child.

 * If cfs_rq is not on the list, test whether a child needs its to be added to

 * connect a branch to the tree  * (see list_add_leaf_cfs_rq() for details).

 */

static inline bool child_cfs_rq_on_list(struct cfs_rq *cfs_rq)

{

	struct cfs_rq *prev_cfs_rq;

	struct list_head *prev;



	if (cfs_rq->on_list) {

		prev = cfs_rq->leaf_cfs_rq_list.prev;

	} else {

		struct rq *rq = rq_of(cfs_rq);



		prev = rq->tmp_alone_branch;

	}



	prev_cfs_rq = container_of(prev, struct cfs_rq, leaf_cfs_rq_list);



	return (prev_cfs_rq->tg->parent == cfs_rq->tg);

}



static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)

{
@@ -3313,6 +3338,9 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq) 
	if (cfs_rq->avg.runnable_sum)

		return false;



	if (child_cfs_rq_on_list(cfs_rq))

		return false;



	return true;

}