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]>
---
v2: Fix build error on !CONFIG_SMP
kernel/sched/fair.c | 43 +++++++++++++++++++++++++++++++++----------
1 file changed, 33 insertions(+), 10 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 384900bf87eb..3d1886ea18fe 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5743,13 +5743,16 @@ 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 rq_flags rf;
struct rq *rq;
+#ifdef CONFIG_SMP
+ struct cfs_rq *tmp;
+ LIST_HEAD(local_unthrottle);
+#endif
rcu_read_lock();
list_for_each_entry_rcu(cfs_rq, &cfs_b->throttled_cfs_rq,
@@ -5786,11 +5789,21 @@ 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))
+#ifdef CONFIG_SMP
+ 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
+ unthrottle_cfs_rq_async(cfs_rq);
+#endif
} else {
throttled = true;
}
@@ -5798,15 +5811,25 @@ 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);
+#ifdef CONFIG_SMP
+ 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));
+#endif
+
+ rcu_read_unlock();
return throttled;
}
--
2.42.0.515.g380fc7ccd1-goog
* 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]>
> ---
> v2: Fix build error on !CONFIG_SMP
>
> kernel/sched/fair.c | 43 +++++++++++++++++++++++++++++++++----------
> 1 file changed, 33 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 384900bf87eb..3d1886ea18fe 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5743,13 +5743,16 @@ 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 rq_flags rf;
> struct rq *rq;
> +#ifdef CONFIG_SMP
> + struct cfs_rq *tmp;
> + LIST_HEAD(local_unthrottle);
> +#endif
>
> rcu_read_lock();
> list_for_each_entry_rcu(cfs_rq, &cfs_b->throttled_cfs_rq,
> @@ -5786,11 +5789,21 @@ 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))
> +#ifdef CONFIG_SMP
> + 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
> + unthrottle_cfs_rq_async(cfs_rq);
> +#endif
> } else {
> throttled = true;
> }
> @@ -5798,15 +5811,25 @@ 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);
> +#ifdef CONFIG_SMP
> + 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));
> +#endif
So instead of uglifying the code with seldom-tested !CONFIG_SMP #ifdefs,
please fold the !SMP code into SMP code: make ->throttled_csd_list
unconditional in a preparatory patch (even if it's essentially unused on
!SMP), then add what is basically your v1 patch as a second patch in the
series to fix the bug.
We want to unify as much of the SMP and !SMP codepaths as possible, even
when it casuses a bit of runtime overhead for !SMP.
Thanks,
Ingo
On Fri, Sep 22, 2023 at 1:13 AM Ingo Molnar <[email protected]> wrote:
>
> So instead of uglifying the code with seldom-tested !CONFIG_SMP #ifdefs,
> please fold the !SMP code into SMP code: make ->throttled_csd_list
> unconditional in a preparatory patch (even if it's essentially unused on
> !SMP), then add what is basically your v1 patch as a second patch in the
> series to fix the bug.
>
> We want to unify as much of the SMP and !SMP codepaths as possible, even
> when it casuses a bit of runtime overhead for !SMP.
Yep, makes perfect sense, I'll respin this patch.
Thanks,
Josh