[RFC PATCH] sched: fix hierarchical order in rq->leaf_cfs_rq_list

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[RFC PATCH] sched: fix hierarchical order in rq->leaf_cfs_rq_list

Vincent Guittot
Fix the insertion of cfs_rq in rq->leaf_cfs_rq_list to ensure that
a child will always called before its parent.

The hierarchical order in shares update list has been introduced by
commit 67e86250f8ea ("sched: Introduce hierarchal order on shares update list")

With the current implementation a child can be still put after its parent.

Lets take the example of
       root
         \
          b
          /\
          c d*
            |
            e*

with root -> b -> c already enqueued but not d -> e so the leaf_cfs_rq_list
looks like: head -> c -> b -> root -> tail

The branch d -> e will be added the first time that they are enqueued,
starting with e then d.

When e is added, its parents is not already on the list so e is put at the
tail : head -> c -> b -> root -> e -> tail

Then, d is added at the head because its parent is already on the list:
head -> d -> c -> b -> root -> e -> tail

e is not placed at the right position and will be called the last whereas
it should be called at the beginning.

Because it follows the bottom-up enqueue sequence, we are sure that we
will finished to add either a cfs_rq without parent or a cfs_rq with a parent
that is already on the list. We can use this event to detect when we have
finished to add a new branch. For the others, whose parents are not already
added, we have to ensure that they will be added after their children that
have just been inserted the steps before, and after any potential parents that
are already in the list. The easiest way is to put the cfs_rq just after the
last inserted one and to keep track of it untl the branch is fully added.

Signed-off-by: Vincent Guittot <[hidden email]>
---
 kernel/sched/core.c  |  1 +
 kernel/sched/fair.c  | 24 ++++++++++++++++++++----
 kernel/sched/sched.h |  1 +
 3 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index adcafda..ef97be4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7351,6 +7351,7 @@ void __init sched_init(void)
 #ifdef CONFIG_FAIR_GROUP_SCHED
  root_task_group.shares = ROOT_TASK_GROUP_LOAD;
  INIT_LIST_HEAD(&rq->leaf_cfs_rq_list);
+ rq->leaf_alone = &rq->leaf_cfs_rq_list;
  /*
  * How much cpu bandwidth does root_task_group get?
  *
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b8a33ab..07f0f1b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -290,15 +290,31 @@ static inline void list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
  * Ensure we either appear before our parent (if already
  * enqueued) or force our parent to appear after us when it is
  * enqueued.  The fact that we always enqueue bottom-up
- * reduces this to two cases.
+ * reduces this to two cases and a special case for the root
+ * cfs_rq.
  */
  if (cfs_rq->tg->parent &&
     cfs_rq->tg->parent->cfs_rq[cpu_of(rq_of(cfs_rq))]->on_list) {
- list_add_rcu(&cfs_rq->leaf_cfs_rq_list,
- &rq_of(cfs_rq)->leaf_cfs_rq_list);
- } else {
+ /* Add the child just before its parent */
+ list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
+ &(cfs_rq->tg->parent->cfs_rq[cpu_of(rq_of(cfs_rq))]->leaf_cfs_rq_list));
+ rq_of(cfs_rq)->leaf_alone = &rq_of(cfs_rq)->leaf_cfs_rq_list;
+ } else if (!cfs_rq->tg->parent) {
+ /*
+ * cfs_rq without parent should be
+ * at the end of the list
+ */
  list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
  &rq_of(cfs_rq)->leaf_cfs_rq_list);
+ rq_of(cfs_rq)->leaf_alone = &rq_of(cfs_rq)->leaf_cfs_rq_list;
+ } else {
+ /*
+ * Our parent has not already been added so make sure
+ * that it will be put after us
+ */
+ list_add_rcu(&cfs_rq->leaf_cfs_rq_list,
+ rq_of(cfs_rq)->leaf_alone);
+ rq_of(cfs_rq)->leaf_alone = &cfs_rq->leaf_cfs_rq_list;
  }
 
  cfs_rq->on_list = 1;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 69da6fc..9693fe9 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -607,6 +607,7 @@ struct rq {
 #ifdef CONFIG_FAIR_GROUP_SCHED
  /* list of leaf cfs_rq on this cpu: */
  struct list_head leaf_cfs_rq_list;
+ struct list_head *leaf_alone;
 #endif /* CONFIG_FAIR_GROUP_SCHED */
 
  /*
--
1.9.1

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[RFC PATCH] sched: fix hierarchical order in rq->leaf_cfs_rq_list

Vincent Guittot
Fix the insertion of cfs_rq in rq->leaf_cfs_rq_list to ensure that
a child will always called before its parent.

The hierarchical order in shares update list has been introduced by
commit 67e86250f8ea ("sched: Introduce hierarchal order on shares update list")

With the current implementation a child can be still put after its parent.

Lets take the example of
       root
         \
          b
          /\
          c d*
            |
            e*

with root -> b -> c already enqueued but not d -> e so the leaf_cfs_rq_list
looks like: head -> c -> b -> root -> tail

The branch d -> e will be added the first time that they are enqueued,
starting with e then d.

When e is added, its parents is not already on the list so e is put at the
tail : head -> c -> b -> root -> e -> tail

Then, d is added at the head because its parent is already on the list:
head -> d -> c -> b -> root -> e -> tail

e is not placed at the right position and will be called the last whereas
it should be called at the beginning.

Because it follows the bottom-up enqueue sequence, we are sure that we
will finished to add either a cfs_rq without parent or a cfs_rq with a parent
that is already on the list. We can use this event to detect when we have
finished to add a new branch. For the others, whose parents are not already
added, we have to ensure that they will be added after their children that
have just been inserted the steps before, and after any potential parents that
are already in the list. The easiest way is to put the cfs_rq just after the
last inserted one and to keep track of it untl the branch is fully added.

Signed-off-by: Vincent Guittot <[hidden email]>
---

Hi,

Same version but rebased on latest sched/core

Regards,
Vincent

 kernel/sched/core.c  |  1 +
 kernel/sched/fair.c  | 24 ++++++++++++++++++++----
 kernel/sched/sched.h |  1 +
 3 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 404c078..7b0fbdc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7386,6 +7386,7 @@ void __init sched_init(void)
 #ifdef CONFIG_FAIR_GROUP_SCHED
  root_task_group.shares = ROOT_TASK_GROUP_LOAD;
  INIT_LIST_HEAD(&rq->leaf_cfs_rq_list);
+ rq->leaf_alone = &rq->leaf_cfs_rq_list;
  /*
  * How much cpu bandwidth does root_task_group get?
  *
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 218f8e8..6d3fbf2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -290,15 +290,31 @@ static inline void list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
  * Ensure we either appear before our parent (if already
  * enqueued) or force our parent to appear after us when it is
  * enqueued.  The fact that we always enqueue bottom-up
- * reduces this to two cases.
+ * reduces this to two cases and a specila case for the root
+ * cfs_rq.
  */
  if (cfs_rq->tg->parent &&
     cfs_rq->tg->parent->cfs_rq[cpu_of(rq_of(cfs_rq))]->on_list) {
- list_add_rcu(&cfs_rq->leaf_cfs_rq_list,
- &rq_of(cfs_rq)->leaf_cfs_rq_list);
- } else {
+ /* Add the child just before its parent */
+ list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
+ &(cfs_rq->tg->parent->cfs_rq[cpu_of(rq_of(cfs_rq))]->leaf_cfs_rq_list));
+ rq_of(cfs_rq)->leaf_alone = &rq_of(cfs_rq)->leaf_cfs_rq_list;
+ } else if (!cfs_rq->tg->parent) {
+ /*
+ * cfs_rq without parent should be
+ * at the end of the list
+ */
  list_add_tail_rcu(&cfs_rq->leaf_cfs_rq_list,
  &rq_of(cfs_rq)->leaf_cfs_rq_list);
+ rq_of(cfs_rq)->leaf_alone = &rq_of(cfs_rq)->leaf_cfs_rq_list;
+ } else {
+ /*
+ * Our parent has not already been added so make sure
+ * that it will be put after us
+ */
+ list_add_rcu(&cfs_rq->leaf_cfs_rq_list,
+ rq_of(cfs_rq)->leaf_alone);
+ rq_of(cfs_rq)->leaf_alone = &cfs_rq->leaf_cfs_rq_list;
  }
 
  cfs_rq->on_list = 1;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e51145e..30750dc 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -614,6 +614,7 @@ struct rq {
 #ifdef CONFIG_FAIR_GROUP_SCHED
  /* list of leaf cfs_rq on this cpu: */
  struct list_head leaf_cfs_rq_list;
+ struct list_head *leaf_alone;
 #endif /* CONFIG_FAIR_GROUP_SCHED */
 
  /*
--
1.9.1

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Re: [RFC PATCH] sched: fix hierarchical order in rq->leaf_cfs_rq_list

Dietmar Eggemann
Hi Vincent,

On 24/05/16 10:55, Vincent Guittot wrote:

> Fix the insertion of cfs_rq in rq->leaf_cfs_rq_list to ensure that
> a child will always called before its parent.
>
> The hierarchical order in shares update list has been introduced by
> commit 67e86250f8ea ("sched: Introduce hierarchal order on shares update list")
>
> With the current implementation a child can be still put after its parent.
>
> Lets take the example of
>        root
>          \
>           b
>           /\
>           c d*
>             |
>             e*
>
> with root -> b -> c already enqueued but not d -> e so the leaf_cfs_rq_list
> looks like: head -> c -> b -> root -> tail
>
> The branch d -> e will be added the first time that they are enqueued,
> starting with e then d.
>
> When e is added, its parents is not already on the list so e is put at the
> tail : head -> c -> b -> root -> e -> tail
>
> Then, d is added at the head because its parent is already on the list:
> head -> d -> c -> b -> root -> e -> tail
>
> e is not placed at the right position and will be called the last whereas
> it should be called at the beginning.
>
> Because it follows the bottom-up enqueue sequence, we are sure that we
> will finished to add either a cfs_rq without parent or a cfs_rq with a parent
> that is already on the list. We can use this event to detect when we have
> finished to add a new branch. For the others, whose parents are not already
> added, we have to ensure that they will be added after their children that
> have just been inserted the steps before, and after any potential parents that
> are already in the list. The easiest way is to put the cfs_rq just after the
> last inserted one and to keep track of it untl the branch is fully added.
>
> Signed-off-by: Vincent Guittot <[hidden email]>

So the use case for this would be you create a multi-level task group
hierarchy on a cpu (e.g. tg_css_id=2,4,6 on cpu=1) and let a task run in
the highest level task group (tg_css_id=6).

list_add_leaf_cfs_rq() call:

...
enqueue_task_fair: cpu=1 tg_css_id=6 cfs_rq=0xffffffc97500f700 on_list=0
enqueue_task_fair: cpu=1 tg_css_id=4 cfs_rq=0xffffffc975175900 on_list=0
enqueue_task_fair: cpu=1 tg_css_id=2 cfs_rq=0xffffffc975866d00 on_list=0
enqueue_task_fair: cpu=1 tg_css_id=1 cfs_rq=0xffffffc97fec44e8 on_list=1

...

In this case, the for_each_leaf_cfs_rq() in update_blocked_averages()
iterates in the tg_css_id=2,1,6,4 order:

...
update_blocked_averages: tg_css_id=2 cfs_rq=0xffffffc975866d00 on_list=1
update_blocked_averages: tg_css_id=1 cfs_rq=0xffffffc97fec44e8 on_list=1
update_blocked_averages: tg_css_id=6 cfs_rq=0xffffffc97500f700 on_list=1
update_blocked_averages: tg_css_id=4 cfs_rq=0xffffffc975175900 on_list=1
...

which I guess breaks the update_tg_cfs_util() call you introduced in
update_blocked_averages() in '[RFC PATCH v2] sched: reflect sched_entity
movement into task_group's utilization'. IMHO, otherwise
update_blocked_averages() can deal with the list not being ordered
tg_css_id=6,4,2,1.

https://lkml.org/lkml/2016/5/24/200

The use of for_each_leaf_cfs_rq() in update_shares() is gone. Do the
remaining call sites (update_runtime_enabled(),
unthrottle_offline_cfs_rqs() require any ordering?

[...]
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Re: [RFC PATCH] sched: fix hierarchical order in rq->leaf_cfs_rq_list

Vincent Guittot
On 25 May 2016 at 19:40, Dietmar Eggemann <[hidden email]> wrote:

> Hi Vincent,
>
> On 24/05/16 10:55, Vincent Guittot wrote:
>> Fix the insertion of cfs_rq in rq->leaf_cfs_rq_list to ensure that
>> a child will always called before its parent.
>>
>> The hierarchical order in shares update list has been introduced by
>> commit 67e86250f8ea ("sched: Introduce hierarchal order on shares update list")
>>
>> With the current implementation a child can be still put after its parent.
>>
>> Lets take the example of
>>        root
>>          \
>>           b
>>           /\
>>           c d*
>>             |
>>             e*
>>
>> with root -> b -> c already enqueued but not d -> e so the leaf_cfs_rq_list
>> looks like: head -> c -> b -> root -> tail
>>
>> The branch d -> e will be added the first time that they are enqueued,
>> starting with e then d.
>>
>> When e is added, its parents is not already on the list so e is put at the
>> tail : head -> c -> b -> root -> e -> tail
>>
>> Then, d is added at the head because its parent is already on the list:
>> head -> d -> c -> b -> root -> e -> tail
>>
>> e is not placed at the right position and will be called the last whereas
>> it should be called at the beginning.
>>
>> Because it follows the bottom-up enqueue sequence, we are sure that we
>> will finished to add either a cfs_rq without parent or a cfs_rq with a parent
>> that is already on the list. We can use this event to detect when we have
>> finished to add a new branch. For the others, whose parents are not already
>> added, we have to ensure that they will be added after their children that
>> have just been inserted the steps before, and after any potential parents that
>> are already in the list. The easiest way is to put the cfs_rq just after the
>> last inserted one and to keep track of it untl the branch is fully added.
>>
>> Signed-off-by: Vincent Guittot <[hidden email]>
>
> So the use case for this would be you create a multi-level task group
> hierarchy on a cpu (e.g. tg_css_id=2,4,6 on cpu=1) and let a task run in
> the highest level task group (tg_css_id=6).
>
> list_add_leaf_cfs_rq() call:
>
> ...
> enqueue_task_fair: cpu=1 tg_css_id=6 cfs_rq=0xffffffc97500f700 on_list=0
> enqueue_task_fair: cpu=1 tg_css_id=4 cfs_rq=0xffffffc975175900 on_list=0
> enqueue_task_fair: cpu=1 tg_css_id=2 cfs_rq=0xffffffc975866d00 on_list=0
> enqueue_task_fair: cpu=1 tg_css_id=1 cfs_rq=0xffffffc97fec44e8 on_list=1
>
> ...
>
> In this case, the for_each_leaf_cfs_rq() in update_blocked_averages()
> iterates in the tg_css_id=2,1,6,4 order:
>
> ...
> update_blocked_averages: tg_css_id=2 cfs_rq=0xffffffc975866d00 on_list=1
> update_blocked_averages: tg_css_id=1 cfs_rq=0xffffffc97fec44e8 on_list=1
> update_blocked_averages: tg_css_id=6 cfs_rq=0xffffffc97500f700 on_list=1
> update_blocked_averages: tg_css_id=4 cfs_rq=0xffffffc975175900 on_list=1
> ...
>
> which I guess breaks the update_tg_cfs_util() call you introduced in
> update_blocked_averages() in '[RFC PATCH v2] sched: reflect sched_entity
> movement into task_group's utilization'. IMHO, otherwise
> update_blocked_averages() can deal with the list not being ordered
> tg_css_id=6,4,2,1.
>
> https://lkml.org/lkml/2016/5/24/200
>
> The use of for_each_leaf_cfs_rq() in update_shares() is gone. Do the
> remaining call sites (update_runtime_enabled(),
> unthrottle_offline_cfs_rqs() require any ordering?

I can see one potential issue with unthrottle_offline_cfs_rqs() which
goes to the hierarchy when unthrottling.
Otherwise i don't see any other dependency with ordering the cfs_rq
than the patch i have sent for task group utilization
The other point is this patch align the code with the comment

>
> [...]