[PATCH v5] sched/fair: Correctly insert cfs_rq's to list on unthrottle

[PATCH v5] sched/fair: Correctly insert cfs_rq's to list on unthrottle

[Odin Ugedal]

This fixes 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 happen 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: 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
Signed-off-by: Odin Ugedal <odin@uged.al>
---
Changes since v1:
 - Replaced cfs_rq field with using tg_load_avg_contrib
 - Went from 3 to 1 patches; one is merged and one is replaced
   by a new patchset.
Changes since v2:
 - Use !cfs_rq_is_decayed() instead of tg_load_avg_contrib
 - Moved cfs_rq_is_decayed to above its new use
Changes since v3:
 - (hopefully) Fix config for !CONFIG_SMP
Changes since v4:
 - Move cfs_rq_is_decayed again

 kernel/sched/fair.c | 39 ++++++++++++++++++++-------------------
 1 file changed, 20 insertions(+), 19 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 794c2cb945f8..c48d1d409b20 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3283,6 +3283,24 @@ 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
@@ -4719,8 +4737,8 @@ 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);
 	}
 
@@ -7895,23 +7913,6 @@ 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;
-- 
2.31.1

Re: [PATCH v5] sched/fair: Correctly insert cfs_rq's to list on unthrottle

[Vincent Guittot]

On Sat, 12 Jun 2021 at 13:31, Odin Ugedal <odin@uged.al> wrote:
>
> This fixes 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 happen because when a cfs_rq is throttled, all the descendant cfs_rq's

s/happen/happens/

> 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: 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
> Signed-off-by: Odin Ugedal <odin@uged.al>

minor typo in the commit message otherwise

Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>

> ---
> Changes since v1:
>  - Replaced cfs_rq field with using tg_load_avg_contrib
>  - Went from 3 to 1 patches; one is merged and one is replaced
>    by a new patchset.
> Changes since v2:
>  - Use !cfs_rq_is_decayed() instead of tg_load_avg_contrib
>  - Moved cfs_rq_is_decayed to above its new use
> Changes since v3:
>  - (hopefully) Fix config for !CONFIG_SMP
> Changes since v4:
>  - Move cfs_rq_is_decayed again
>
>  kernel/sched/fair.c | 39 ++++++++++++++++++++-------------------
>  1 file changed, 20 insertions(+), 19 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 794c2cb945f8..c48d1d409b20 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3283,6 +3283,24 @@ 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
> @@ -4719,8 +4737,8 @@ 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);
>         }
>
> @@ -7895,23 +7913,6 @@ 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;
> --
> 2.31.1
>

Re: [PATCH v5] sched/fair: Correctly insert cfs_rq's to list on unthrottle

[Peter Zijlstra]

On Mon, Jun 14, 2021 at 02:01:55PM +0200, Vincent Guittot wrote:
> On Sat, 12 Jun 2021 at 13:31, Odin Ugedal <odin@uged.al> wrote:
> >
> > This fixes 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 happen because when a cfs_rq is throttled, all the descendant cfs_rq's
> 
> s/happen/happens/
> 
> > 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: 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
> > Signed-off-by: Odin Ugedal <odin@uged.al>
> 
> minor typo in the commit message otherwise
> 
> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>

Thanks!

[tip: sched/urgent] sched/fair: Correctly insert cfs_rq's to list on unthrottle

[tip-bot2 for Odin Ugedal]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     a7b359fc6a37faaf472125867c8dc5a068c90982
Gitweb:        https://git.kernel.org/tip/a7b359fc6a37faaf472125867c8dc5a068c90982
Author:        Odin Ugedal <odin@uged.al>
AuthorDate:    Sat, 12 Jun 2021 13:28:15 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 14 Jun 2021 22:58:47 +02:00

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: 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
[vingo: !SMP build fix]
Signed-off-by: Odin Ugedal <odin@uged.al>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20210612112815.61678-1-odin@uged.al
---
 kernel/sched/fair.c | 44 +++++++++++++++++++++++++-------------------
 1 file changed, 25 insertions(+), 19 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2c8a935..bfaa6e1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -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;