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sched/fair: Fix warning in bandwidth distribution

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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
This commit is contained in:
Josh Don 2023-09-22 16:05:35 -07:00 committed by Ingo Molnar
parent 30797bce8e
commit 2f8c62296b
1 changed files with 25 additions and 11 deletions
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36
kernel/sched/fair.c
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@ -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;
}