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Subject[RFC 52/60] cosched: Support SD-SEs in enqueuing and dequeuing
Date
SD-SEs require some attention during enqueuing and dequeuing. In some
aspects they behave similar to TG-SEs, for example, we must not dequeue
a SD-SE if it still represents other load. But SD-SEs are also different
due to the concurrent load updates by multiple CPUs and that we need to
be careful when to access it, as an SD-SE belongs to the next hierarchy
level which is protected by a different lock.

Make sure to propagate enqueues and dequeues correctly, and to notify
the leader when needed.

Additionally, we define cfs_rq->h_nr_running to refer to number tasks
and SD-SEs below the CFS runqueue without drilling down into SD-SEs.
(Phrased differently, h_nr_running counts non-TG-SEs along the task
group hierarchy.) This makes later adjustments for load-balancing
more natural, as SD-SEs now appear similar to tasks, allowing to
balance coscheduled sets individually.

Signed-off-by: Jan H. Schönherr <jschoenh@amazon.de>
---
kernel/sched/fair.c | 107 +++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 102 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 483db54ee20a..bc219c9c3097 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4600,17 +4600,40 @@ static void throttle_cfs_rq(struct cfs_rq *cfs_rq)
/* throttled entity or throttle-on-deactivate */
if (!se->on_rq)
break;
+ if (is_sd_se(se)) {
+ /*
+ * don't dequeue sd_se if it represents other
+ * children besides the dequeued one
+ */
+ if (se->load.weight)
+ dequeue = 0;
+
+ task_delta = 1;
+ }

if (dequeue)
dequeue_entity(qcfs_rq, se, DEQUEUE_SLEEP);
+ if (dequeue && is_sd_se(se)) {
+ /*
+ * If we dequeued an SD-SE and we are not the leader,
+ * the leader might want to select another task group
+ * right now.
+ *
+ * FIXME: Change leadership instead?
+ */
+ if (leader_of(se) != cpu_of(rq))
+ resched_cpu_locked(leader_of(se));
+ }
+ if (!dequeue && is_sd_se(se))
+ break;
qcfs_rq->h_nr_running -= task_delta;

if (qcfs_rq->load.weight)
dequeue = 0;
}

- if (!se)
- sub_nr_running(rq, task_delta);
+ if (!se || !is_cpu_rq(hrq_of(cfs_rq_of(se))))
+ sub_nr_running(rq, cfs_rq->h_nr_running);

rq_chain_unlock(&rc);

@@ -4641,8 +4664,11 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);
struct sched_entity *se;
int enqueue = 1;
- long task_delta;
+ long task_delta, orig_task_delta;
struct rq_chain rc;
+#ifdef CONFIG_COSCHEDULING
+ int lcpu = rq->sdrq_data.leader;
+#endif

SCHED_WARN_ON(!is_cpu_rq(rq));

@@ -4669,24 +4695,40 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
return;

task_delta = cfs_rq->h_nr_running;
+ orig_task_delta = task_delta;
rq_chain_init(&rc, rq);
for_each_sched_entity(se) {
rq_chain_lock(&rc, se);
update_sdse_load(se);
if (se->on_rq)
enqueue = 0;
+ if (is_sd_se(se))
+ task_delta = 1;

cfs_rq = cfs_rq_of(se);
if (enqueue)
enqueue_entity(cfs_rq, se, ENQUEUE_WAKEUP);
+ if (!enqueue && is_sd_se(se))
+ break;
cfs_rq->h_nr_running += task_delta;

if (cfs_rq_throttled(cfs_rq))
break;
+
+#ifdef CONFIG_COSCHEDULING
+ /*
+ * FIXME: Pro-actively reschedule the leader, can't tell
+ * currently whether we actually have to.
+ */
+ if (lcpu != cfs_rq->sdrq.data->leader) {
+ lcpu = cfs_rq->sdrq.data->leader;
+ resched_cpu_locked(lcpu);
+ }
+#endif /* CONFIG_COSCHEDULING */
}

- if (!se)
- add_nr_running(rq, task_delta);
+ if (!se || !is_cpu_rq(hrq_of(cfs_rq_of(se))))
+ add_nr_running(rq, orig_task_delta);

rq_chain_unlock(&rc);

@@ -5213,6 +5255,9 @@ bool enqueue_entity_fair(struct rq *rq, struct sched_entity *se, int flags,
{
struct cfs_rq *cfs_rq;
struct rq_chain rc;
+#ifdef CONFIG_COSCHEDULING
+ int lcpu = rq->sdrq_data.leader;
+#endif

rq_chain_init(&rc, rq);
for_each_sched_entity(se) {
@@ -5221,6 +5266,8 @@ bool enqueue_entity_fair(struct rq *rq, struct sched_entity *se, int flags,
if (se->on_rq)
break;
cfs_rq = cfs_rq_of(se);
+ if (is_sd_se(se))
+ task_delta = 1;
enqueue_entity(cfs_rq, se, flags);

/*
@@ -5234,6 +5281,22 @@ bool enqueue_entity_fair(struct rq *rq, struct sched_entity *se, int flags,
cfs_rq->h_nr_running += task_delta;

flags = ENQUEUE_WAKEUP;
+
+#ifdef CONFIG_COSCHEDULING
+ /*
+ * FIXME: Pro-actively reschedule the leader, can't tell
+ * currently whether we actually have to.
+ *
+ * There are some cases that slip through
+ * check_preempt_curr(), like the leader not getting
+ * notified (and not becoming aware of the addition
+ * timely), when an RT task is running.
+ */
+ if (lcpu != cfs_rq->sdrq.data->leader) {
+ lcpu = cfs_rq->sdrq.data->leader;
+ resched_cpu_locked(lcpu);
+ }
+#endif /* CONFIG_COSCHEDULING */
}

for_each_sched_entity(se) {
@@ -5241,6 +5304,9 @@ bool enqueue_entity_fair(struct rq *rq, struct sched_entity *se, int flags,
rq_chain_lock(&rc, se);
update_sdse_load(se);
cfs_rq = cfs_rq_of(se);
+
+ if (is_sd_se(se))
+ task_delta = 0;
cfs_rq->h_nr_running += task_delta;

if (cfs_rq_throttled(cfs_rq))
@@ -5304,8 +5370,36 @@ bool dequeue_entity_fair(struct rq *rq, struct sched_entity *se, int flags,
rq_chain_lock(&rc, se);
update_sdse_load(se);
cfs_rq = cfs_rq_of(se);
+
+ if (is_sd_se(se)) {
+ /*
+ * don't dequeue sd_se if it represents other
+ * children besides the dequeued one
+ */
+ if (se->load.weight)
+ break;
+
+ /* someone else did our job */
+ if (!se->on_rq)
+ break;
+
+ task_delta = 1;
+ }
+
dequeue_entity(cfs_rq, se, flags);

+ if (is_sd_se(se)) {
+ /*
+ * If we dequeued an SD-SE and we are not the leader,
+ * the leader might want to select another task group
+ * right now.
+ *
+ * FIXME: Change leadership instead?
+ */
+ if (leader_of(se) != cpu_of(rq))
+ resched_cpu_locked(leader_of(se));
+ }
+
/*
* end evaluation on encountering a throttled cfs_rq
*
@@ -5339,6 +5433,9 @@ bool dequeue_entity_fair(struct rq *rq, struct sched_entity *se, int flags,
rq_chain_lock(&rc, se);
update_sdse_load(se);
cfs_rq = cfs_rq_of(se);
+
+ if (is_sd_se(se))
+ task_delta = 0;
cfs_rq->h_nr_running -= task_delta;

if (cfs_rq_throttled(cfs_rq))
--
2.9.3.1.gcba166c.dirty
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