Messages in this thread Patch in this message | | | From | Michal Koutný <> | Subject | [RFC PATCH] sched: Simplify leaf_cfs_rq_list maintenance | Date | Thu, 25 Feb 2021 17:27:57 +0100 |
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leaf_cfs_rq_list should contain cfs_rqs that have runnable entities in them. When we're operating under a throttled hierarchy we always update the leaf_cfs_rq_list in order no to break list_add_leaf_cfs_rq invariant of adding complete branches.
This caused troubles when an entity became runnable (enqueue_entity) under a throttled hierarchy (see commit b34cb07dde7c ("sched/fair: Fix enqueue_task_fair() warning some more")). While it proved well, this new change ignores updating leaf_cfs_rq_list when we're operating under the throttled hierarchy and defers the leaf_cfs_rq_list update to the point when whole hiearchy is unthrottled (tg_unthrottle_up).
The code is now simpler and leaf_cfs_rq_list contains only cfs_rqs that are truly runnable.
Why is this RFC? - Primarily, I'm not sure I interpreted the purpose of leaf_cfs_rq_list right. The removal of throttled cfs_rqs from it would exclude them from __update_blocked_fair() calculation and I can't see past it now. If it's wrong assumption, I'd like this to help clarify what the proper definition of leaf_cfs_rq_list would be. - Additionally, I didn't check thoroughly for corner cases when se->on_rq => cfs_rq_of(se)->on_list wouldn't hold, so the patch certainly isn't finished.
Signed-off-by: Michal Koutný <mkoutny@suse.com> --- kernel/sched/fair.c | 41 ++++++----------------------------------- kernel/sched/sched.h | 4 +--- 2 files changed, 7 insertions(+), 38 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 04a3ce20da67..634ba6637824 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4250,10 +4250,11 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) /* * When bandwidth control is enabled, cfs might have been removed - * because of a parent been throttled but cfs->nr_running > 1. Try to - * add it unconditionnally. + * because of a parent been throttled. We'll add it later (with + * complete branch up to se->on_rq/cfs_eq->on_list) in + * tg_unthrottle_up() and unthrottle_cfs_rq(). */ - if (cfs_rq->nr_running == 1 || cfs_bandwidth_used()) + if (cfs_rq->nr_running == 1 && !throttled_hierarchy(cfs_rq)) list_add_leaf_cfs_rq(cfs_rq); if (cfs_rq->nr_running == 1) @@ -4859,6 +4860,7 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq) for_each_sched_entity(se) { if (se->on_rq) break; + // XXX: se->on_rq implies cfs_rq_of(se)->on_list (unless throttled_hierarchy) cfs_rq = cfs_rq_of(se); enqueue_entity(cfs_rq, se, ENQUEUE_WAKEUP); @@ -4896,17 +4898,6 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq) add_nr_running(rq, task_delta); unthrottle_throttle: - /* - * The cfs_rq_throttled() breaks in the above iteration can result in - * incomplete leaf list maintenance, resulting in triggering the - * assertion below. - */ - for_each_sched_entity(se) { - cfs_rq = cfs_rq_of(se); - - if (list_add_leaf_cfs_rq(cfs_rq)) - break; - } assert_list_leaf_cfs_rq(rq); @@ -5518,6 +5509,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) for_each_sched_entity(se) { if (se->on_rq) break; + // XXX: se->on_rq implies cfs_rq_of(se)->on_list (unless throttled_hierarchy) cfs_rq = cfs_rq_of(se); enqueue_entity(cfs_rq, se, flags); @@ -5544,13 +5536,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) /* end evaluation on encountering a throttled cfs_rq */ if (cfs_rq_throttled(cfs_rq)) goto enqueue_throttle; - - /* - * One parent has been throttled and cfs_rq removed from the - * list. Add it back to not break the leaf list. - */ - if (throttled_hierarchy(cfs_rq)) - list_add_leaf_cfs_rq(cfs_rq); } /* At this point se is NULL and we are at root level*/ @@ -5574,20 +5559,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) update_overutilized_status(rq); enqueue_throttle: - if (cfs_bandwidth_used()) { - /* - * When bandwidth control is enabled; the cfs_rq_throttled() - * breaks in the above iteration can result in incomplete - * leaf list maintenance, resulting in triggering the assertion - * below. - */ - for_each_sched_entity(se) { - cfs_rq = cfs_rq_of(se); - - if (list_add_leaf_cfs_rq(cfs_rq)) - break; - } - } assert_list_leaf_cfs_rq(rq); diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index bb09988451a0..f674d88920da 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -573,9 +573,7 @@ struct cfs_rq { struct rq *rq; /* CPU runqueue to which this cfs_rq is attached */ /* - * leaf cfs_rqs are those that hold tasks (lowest schedulable entity in - * a hierarchy). Non-leaf lrqs hold other higher schedulable entities - * (like users, containers etc.) + * leaf cfs_rqs are those that hold runnable entities. * * leaf_cfs_rq_list ties together list of leaf cfs_rq's in a CPU. * This list is used during load balance. -- 2.30.1
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