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 <[email protected]>
---
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
