[PATCH v3] sched/fair : fix reordering of enqueue/dequeue_task_fair

[PATCH v3] sched/fair : fix reordering of enqueue/dequeue_task_fair

[Vincent Guittot]

Even when a cgroup is throttled, the group se of a child cgroup can still
be enqueued and its gse->on_rq stays true. When a task is enqueued on such
child, we still have to update the load_avg and increase
h_nr_running of the throttled cfs. Nevertheless, the 1st
for_each_sched_entity loop is skipped because of gse->on_rq == true and the
2nd loop because the cfs is throttled whereas we have to update both
load_avg with the old h_nr_running and increase h_nr_running in such case.

The same sequence can happen during dequeue when se moves to parent before
breaking in the 1st loop.

Note that the update of load_avg will effectively happen only once in order
to sync up to the throttled time. Next call for updating load_avg will stop
early because the clock stays unchanged.

Fixes: 6d4d22468dae ("sched/fair: Reorder enqueue/dequeue_task_fair path")
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---

Changes since v2:
- added similar changes into dequeue_task_fair as reported by Ben

 kernel/sched/fair.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fcc968669aea..ea2748a132a2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5431,16 +5431,16 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	for_each_sched_entity(se) {
 		cfs_rq = cfs_rq_of(se);
 
-		/* end evaluation on encountering a throttled cfs_rq */
-		if (cfs_rq_throttled(cfs_rq))
-			goto enqueue_throttle;
-
 		update_load_avg(cfs_rq, se, UPDATE_TG);
 		se_update_runnable(se);
 		update_cfs_group(se);
 
 		cfs_rq->h_nr_running++;
 		cfs_rq->idle_h_nr_running += idle_h_nr_running;
+
+		/* end evaluation on encountering a throttled cfs_rq */
+		if (cfs_rq_throttled(cfs_rq))
+			goto enqueue_throttle;
 	}
 
 enqueue_throttle:
@@ -5529,16 +5529,17 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	for_each_sched_entity(se) {
 		cfs_rq = cfs_rq_of(se);
 
-		/* end evaluation on encountering a throttled cfs_rq */
-		if (cfs_rq_throttled(cfs_rq))
-			goto dequeue_throttle;
-
 		update_load_avg(cfs_rq, se, UPDATE_TG);
 		se_update_runnable(se);
 		update_cfs_group(se);
 
 		cfs_rq->h_nr_running--;
 		cfs_rq->idle_h_nr_running -= idle_h_nr_running;
+
+		/* end evaluation on encountering a throttled cfs_rq */
+		if (cfs_rq_throttled(cfs_rq))
+			goto dequeue_throttle;
+
 	}
 
 dequeue_throttle:
-- 
2.17.1

[tip: sched/core] sched/fair: Fix reordering of enqueue/dequeue_task_fair()

[tip-bot2 for Vincent Guittot]

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

Commit-ID:     5ab297bab984310267734dfbcc8104566658ebef
Gitweb:        https://git.kernel.org/tip/5ab297bab984310267734dfbcc8104566658ebef
Author:        Vincent Guittot <vincent.guittot@linaro.org>
AuthorDate:    Fri, 06 Mar 2020 09:42:08 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Fri, 06 Mar 2020 12:57:25 +01:00

sched/fair: Fix reordering of enqueue/dequeue_task_fair()

Even when a cgroup is throttled, the group se of a child cgroup can still
be enqueued and its gse->on_rq stays true. When a task is enqueued on such
child, we still have to update the load_avg and increase
h_nr_running of the throttled cfs. Nevertheless, the 1st
for_each_sched_entity() loop is skipped because of gse->on_rq == true and the
2nd loop because the cfs is throttled whereas we have to update both
load_avg with the old h_nr_running and increase h_nr_running in such case.

The same sequence can happen during dequeue when se moves to parent before
breaking in the 1st loop.

Note that the update of load_avg will effectively happen only once in order
to sync up to the throttled time. Next call for updating load_avg will stop
early because the clock stays unchanged.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Fixes: 6d4d22468dae ("sched/fair: Reorder enqueue/dequeue_task_fair path")
Link: https://lkml.kernel.org/r/20200306084208.12583-1-vincent.guittot@linaro.org
---
 kernel/sched/fair.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 54bd628..1dea855 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5460,16 +5460,16 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	for_each_sched_entity(se) {
 		cfs_rq = cfs_rq_of(se);
 
-		/* end evaluation on encountering a throttled cfs_rq */
-		if (cfs_rq_throttled(cfs_rq))
-			goto enqueue_throttle;
-
 		update_load_avg(cfs_rq, se, UPDATE_TG);
 		se_update_runnable(se);
 		update_cfs_group(se);
 
 		cfs_rq->h_nr_running++;
 		cfs_rq->idle_h_nr_running += idle_h_nr_running;
+
+		/* end evaluation on encountering a throttled cfs_rq */
+		if (cfs_rq_throttled(cfs_rq))
+			goto enqueue_throttle;
 	}
 
 enqueue_throttle:
@@ -5558,16 +5558,17 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	for_each_sched_entity(se) {
 		cfs_rq = cfs_rq_of(se);
 
-		/* end evaluation on encountering a throttled cfs_rq */
-		if (cfs_rq_throttled(cfs_rq))
-			goto dequeue_throttle;
-
 		update_load_avg(cfs_rq, se, UPDATE_TG);
 		se_update_runnable(se);
 		update_cfs_group(se);
 
 		cfs_rq->h_nr_running--;
 		cfs_rq->idle_h_nr_running -= idle_h_nr_running;
+
+		/* end evaluation on encountering a throttled cfs_rq */
+		if (cfs_rq_throttled(cfs_rq))
+			goto dequeue_throttle;
+
 	}
 
 dequeue_throttle:

Re: [PATCH v3] sched/fair : fix reordering of enqueue/dequeue_task_fair

[bsegall]

Vincent Guittot <vincent.guittot@linaro.org> writes:

> Even when a cgroup is throttled, the group se of a child cgroup can still
> be enqueued and its gse->on_rq stays true. When a task is enqueued on such
> child, we still have to update the load_avg and increase
> h_nr_running of the throttled cfs. Nevertheless, the 1st
> for_each_sched_entity loop is skipped because of gse->on_rq == true and the
> 2nd loop because the cfs is throttled whereas we have to update both
> load_avg with the old h_nr_running and increase h_nr_running in such case.
>
> The same sequence can happen during dequeue when se moves to parent before
> breaking in the 1st loop.
>
> Note that the update of load_avg will effectively happen only once in order
> to sync up to the throttled time. Next call for updating load_avg will stop
> early because the clock stays unchanged.


Reviewed-by: Ben Segall <bsegall@google.com>

(though it seems I was too slow in actually testing this and it's in
tip, which confused me a bunch when I tried to apply the patch for testing)

>
> Fixes: 6d4d22468dae ("sched/fair: Reorder enqueue/dequeue_task_fair path")
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>
> Changes since v2:
> - added similar changes into dequeue_task_fair as reported by Ben
>
>  kernel/sched/fair.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fcc968669aea..ea2748a132a2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5431,16 +5431,16 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	for_each_sched_entity(se) {
>  		cfs_rq = cfs_rq_of(se);
>  
> -		/* end evaluation on encountering a throttled cfs_rq */
> -		if (cfs_rq_throttled(cfs_rq))
> -			goto enqueue_throttle;
> -
>  		update_load_avg(cfs_rq, se, UPDATE_TG);
>  		se_update_runnable(se);
>  		update_cfs_group(se);
>  
>  		cfs_rq->h_nr_running++;
>  		cfs_rq->idle_h_nr_running += idle_h_nr_running;
> +
> +		/* end evaluation on encountering a throttled cfs_rq */
> +		if (cfs_rq_throttled(cfs_rq))
> +			goto enqueue_throttle;
>  	}
>  
>  enqueue_throttle:
> @@ -5529,16 +5529,17 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	for_each_sched_entity(se) {
>  		cfs_rq = cfs_rq_of(se);
>  
> -		/* end evaluation on encountering a throttled cfs_rq */
> -		if (cfs_rq_throttled(cfs_rq))
> -			goto dequeue_throttle;
> -
>  		update_load_avg(cfs_rq, se, UPDATE_TG);
>  		se_update_runnable(se);
>  		update_cfs_group(se);
>  
>  		cfs_rq->h_nr_running--;
>  		cfs_rq->idle_h_nr_running -= idle_h_nr_running;
> +
> +		/* end evaluation on encountering a throttled cfs_rq */
> +		if (cfs_rq_throttled(cfs_rq))
> +			goto dequeue_throttle;
> +
>  	}
>  
>  dequeue_throttle: