[tip: sched/core] sched/fair: Fix warning in bandwidth distribution

tip-bot2 for Josh Don
2023-09-24T10:39:37Z

The following commit has been merged into the sched/core branch of tip:

Commit-ID: 2f8c62296b6f656bbfd17e9f1fadd7478003a9d9
Gitweb: https://git.kernel.org/tip/2f8c62296b6f656bbfd17e9f1fadd7478003a9d9
Author: Josh Don <joshdon@google.com>
AuthorDate: Fri, 22 Sep 2023 16:05:35 -07:00
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Sun, 24 Sep 2023 12:08:29 +02:00

sched/fair: Fix warning in bandwidth distribution

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:

- CPU 0: 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.

- CPU 1: 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>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230922230535.296350-2-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 41c960e..2973173 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5741,13 +5741,13 @@ 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,
@@ -5782,11 +5782,17 @@ 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;
}
@@ -5794,15 +5800,23 @@ 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;
}