[PATCH 2/2] sched: fix warning in bandwidth distribution

Josh Don
2023-09-22T23:06:15Z

We've observed the following warning being hit in
distribute_cfs_runtime():
SCHED_WARN_ON(cfs_rq->runtime_remaining > 0)

We have the following race:

- cpu0: running bandwidth distribution (distribute_cfs_runtime).
Inspects the local cfs_rq and makes its runtime_remaining positive.
However, we defer unthrottling the local cfs_rq until after
considering all remote cfs_rq's.
- cpu1: starts running bandwidth distribution from the slack timer. When
it finds the cfs_rq for cpu 0 on the throttled list, it observers the
that the cfs_rq is throttled, yet is not on the CSD list, and has a
positive runtime_remaining, thus triggering the warning in
distribute_cfs_runtime.

To fix this, we can rework the local unthrottling logic to put the local
cfs_rq on a local list, so that any future bandwidth distributions will
realize that the cfs_rq is about to be unthrottled.

Signed-off-by: Josh Don <joshdon@google.com>
---
kernel/sched/fair.c | 36 +++++++++++++++++++++++++-----------
1 file changed, 25 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8f4e63fc8900..de002dab28cf 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5743,13 +5743,13 @@ static void unthrottle_cfs_rq_async(struct cfs_rq *cfs_rq)

static bool distribute_cfs_runtime(struct cfs_bandwidth *cfs_b)
{
- struct cfs_rq *local_unthrottle = NULL;
int this_cpu = smp_processor_id();
u64 runtime, remaining = 1;
bool throttled = false;
- struct cfs_rq *cfs_rq;
+ struct cfs_rq *cfs_rq, *tmp;
struct rq_flags rf;
struct rq *rq;
+ LIST_HEAD(local_unthrottle);

rcu_read_lock();
list_for_each_entry_rcu(cfs_rq, &cfs_b->throttled_cfs_rq,
@@ -5784,11 +5784,17 @@ static bool distribute_cfs_runtime(struct cfs_bandwidth *cfs_b)

/* we check whether we're throttled above */
if (cfs_rq->runtime_remaining > 0) {
- if (cpu_of(rq) != this_cpu ||
- SCHED_WARN_ON(local_unthrottle))
+ if (cpu_of(rq) != this_cpu) {
unthrottle_cfs_rq_async(cfs_rq);
- else
- local_unthrottle = cfs_rq;
+ } else {
+ /*
+ * We currently only expect to be unthrottling
+ * a single cfs_rq locally.
+ */
+ SCHED_WARN_ON(!list_empty(&local_unthrottle));
+ list_add_tail(&cfs_rq->throttled_csd_list,
+ &local_unthrottle);
+ }
} else {
throttled = true;
}
@@ -5796,15 +5802,23 @@ static bool distribute_cfs_runtime(struct cfs_bandwidth *cfs_b)
next:
rq_unlock_irqrestore(rq, &rf);
}
- rcu_read_unlock();

- if (local_unthrottle) {
- rq = cpu_rq(this_cpu);
+ list_for_each_entry_safe(cfs_rq, tmp, &local_unthrottle,
+ throttled_csd_list) {
+ struct rq *rq = rq_of(cfs_rq);
+
rq_lock_irqsave(rq, &rf);
- if (cfs_rq_throttled(local_unthrottle))
- unthrottle_cfs_rq(local_unthrottle);
+
+ list_del_init(&cfs_rq->throttled_csd_list);
+
+ if (cfs_rq_throttled(cfs_rq))
+ unthrottle_cfs_rq(cfs_rq);
+
rq_unlock_irqrestore(rq, &rf);
}
+ SCHED_WARN_ON(!list_empty(&local_unthrottle));
+
+ rcu_read_unlock();

return throttled;
}
--
2.42.0.515.g380fc7ccd1-goog