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

Ingo Molnar
2023-09-24T10:12:43Z

* Josh Don <joshdon@google.com> 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 <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();

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