Messages in this thread | | | Subject | Re: [RFC PATCH] sched: fix hierarchical order in rq->leaf_cfs_rq_list | From | Dietmar Eggemann <> | Date | Wed, 25 May 2016 18:40:11 +0100 |
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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 <vincent.guittot@linaro.org>
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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