Commit a7b359fc authored by Odin Ugedal's avatar Odin Ugedal Committed by Peter Zijlstra
Browse files

sched/fair: Correctly insert cfs_rq's to list on unthrottle 



Fix an issue where fairness is decreased since cfs_rq's can end up not
being decayed properly. For two sibling control groups with the same
priority, this can often lead to a load ratio of 99/1 (!!).

This happens because when a cfs_rq is throttled, all the descendant
cfs_rq's will be removed from the leaf list. When they initial cfs_rq
is unthrottled, it will currently only re add descendant cfs_rq's if
they have one or more entities enqueued. This is not a perfect
heuristic.

Instead, we insert all cfs_rq's that contain one or more enqueued
entities, or it its load is not completely decayed.

Can often lead to situations like this for equally weighted control
groups:

  $ ps u -C stress
  USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
  root       10009 88.8  0.0   3676   100 pts/1    R+   11:04   0:13 stress --cpu 1
  root       10023  3.0  0.0   3676   104 pts/1    R+   11:04   0:00 stress --cpu 1

Fixes: 31bc6aea ("sched/fair: Optimize update_blocked_averages()")
[vingo: !SMP build fix]
Signed-off-by: default avatarOdin Ugedal <odin@uged.al>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20210612112815.61678-1-odin@uged.al
parent 009c9aa5

kernel/sched/fair.c

+25 −19
Original line number Diff line number Diff line
@@ -3298,6 +3298,24 @@ static inline void cfs_rq_util_change(struct cfs_rq *cfs_rq, int flags) 


#ifdef CONFIG_SMP

#ifdef CONFIG_FAIR_GROUP_SCHED



static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)

{

	if (cfs_rq->load.weight)

		return false;



	if (cfs_rq->avg.load_sum)

		return false;



	if (cfs_rq->avg.util_sum)

		return false;



	if (cfs_rq->avg.runnable_sum)

		return false;



	return true;

}



/**

 * update_tg_load_avg - update the tg's load avg

 * @cfs_rq: the cfs_rq whose avg changed
@@ -4091,6 +4109,11 @@ static inline void update_misfit_status(struct task_struct *p, struct rq *rq) 


#else /* CONFIG_SMP */



static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)

{

	return true;

}



#define UPDATE_TG	0x0

#define SKIP_AGE_LOAD	0x0

#define DO_ATTACH	0x0
@@ -4749,8 +4772,8 @@ static int tg_unthrottle_up(struct task_group *tg, void *data) 
		cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -

					     cfs_rq->throttled_clock_task;



		/* Add cfs_rq with already running entity in the list */

		if (cfs_rq->nr_running >= 1)

		/* Add cfs_rq with load or one or more already running entities to the list */

		if (!cfs_rq_is_decayed(cfs_rq) || cfs_rq->nr_running)

			list_add_leaf_cfs_rq(cfs_rq);

	}
@@ -7996,23 +8019,6 @@ static bool __update_blocked_others(struct rq *rq, bool *done) 


#ifdef CONFIG_FAIR_GROUP_SCHED



static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)

{

	if (cfs_rq->load.weight)

		return false;



	if (cfs_rq->avg.load_sum)

		return false;



	if (cfs_rq->avg.util_sum)

		return false;



	if (cfs_rq->avg.runnable_sum)

		return false;



	return true;

}



static bool __update_blocked_fair(struct rq *rq, bool *done)

{

	struct cfs_rq *cfs_rq, *pos;