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

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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: a7b359fc6a ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
Reported-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
Acked-by: Odin Ugedal <odin@uged.al>
Link: https://lore.kernel.org/r/20210621174330.11258-1-vincent.guittot@linaro.org
This commit is contained in:
Rik van Riel 2021-06-21 19:43:30 +02:00 committed by Ingo Molnar
parent a96bfed64c
commit fdaba61ef8
1 changed files with 28 additions and 0 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

28
kernel/sched/fair.c
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@ -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;
}