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 <[email protected]>
---
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
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 <[email protected]>
AuthorDate: Fri, 22 Sep 2023 16:05:35 -07:00
Committer: Ingo Molnar <[email protected]>
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 <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
---
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;
}
* Josh Don <[email protected]> wrote:
> 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 <[email protected]>
> ---
> 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();
Thanks, this looks much cleaner.
When the warning hits, we don't have any other side-effects,
such as bad behavior or data corruption, correct?
Under that assumption I've queued your fix in tip:sched/core,
for a v6.7 merge, and not in tip:sched/urgent for a v6.6 merge,
but let me know if I'm reading the code wrong...
Thanks,
Ingo
> When the warning hits, we don't have any other side-effects,
> such as bad behavior or data corruption, correct?
Correct, the warning is benign.
> Under that assumption I've queued your fix in tip:sched/core,
> for a v6.7 merge, and not in tip:sched/urgent for a v6.6 merge,
> but let me know if I'm reading the code wrong...
Thanks Ingo!