[PATCH v2] sched/fair: don't assign runtime for throttled cfs_rq

[PATCH v2] sched/fair: don't assign runtime for throttled cfs_rq

[Liangyan]

do_sched_cfs_period_timer() will refill cfs_b runtime and call
distribute_cfs_runtime() to unthrottle cfs_rq, sometimes cfs_b->runtime
will allocate all quota to one cfs_rq incorrectly.
This will cause other cfs_rq can't get runtime and will be throttled.
We find that one throttled cfs_rq has non-negative
cfs_rq->runtime_remaining and cause an unexpetced cast from s64 to u64
in snippet: distribute_cfs_runtime() {
runtime = -cfs_rq->runtime_remaining + 1; }.
This cast will cause that runtime will be a large number and
cfs_b->runtime will be subtracted to be zero at last.
According to Ben Segall, the throttled cfs_rq can have
account_cfs_rq_runtime called on it because it is throttled before
idle_balance, and the idle_balance calls update_rq_clock to add time
that is accounted to the task.

This commit prevents cfs_rq to be assgined new runtime if it has been
throttled to avoid the above incorrect type cast.

Signed-off-by: Liangyan <liangyan.peng@linux.alibaba.com>
Reviewed-by: Ben Segall <bsegall@google.com>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/sched/fair.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bc9cfeaac8bd..ac3ae694d850 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4470,6 +4470,8 @@ static void __account_cfs_rq_runtime(struct cfs_rq *cfs_rq, u64 delta_exec)
 	if (likely(cfs_rq->runtime_remaining > 0))
 		return;
 
+	if (cfs_rq->throttled)
+		return;
 	/*
 	 * if we're unable to extend our runtime we resched so that the active
 	 * hierarchy can be throttled
@@ -4673,6 +4675,9 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
 		if (!cfs_rq_throttled(cfs_rq))
 			goto next;
 
+		/* By the above check, this should never be true */
+		WARN_ON(cfs_rq->runtime_remaining > 0);
+
 		runtime = -cfs_rq->runtime_remaining + 1;
 		if (runtime > remaining)
 			runtime = remaining;
-- 
2.14.4.44.g2045bb6

Re: [PATCH v2] sched/fair: don't assign runtime for throttled cfs_rq

[Peter Zijlstra]

On Sat, Aug 24, 2019 at 05:15:58PM +0800, Liangyan wrote:
> do_sched_cfs_period_timer() will refill cfs_b runtime and call
> distribute_cfs_runtime() to unthrottle cfs_rq, sometimes cfs_b->runtime
> will allocate all quota to one cfs_rq incorrectly.
> This will cause other cfs_rq can't get runtime and will be throttled.
> We find that one throttled cfs_rq has non-negative
> cfs_rq->runtime_remaining and cause an unexpetced cast from s64 to u64
> in snippet: distribute_cfs_runtime() {
> runtime = -cfs_rq->runtime_remaining + 1; }.
> This cast will cause that runtime will be a large number and
> cfs_b->runtime will be subtracted to be zero at last.
> According to Ben Segall, the throttled cfs_rq can have
> account_cfs_rq_runtime called on it because it is throttled before
> idle_balance, and the idle_balance calls update_rq_clock to add time
> that is accounted to the task.

That is distinctly unreadable. Please try again with a little bit of
whitespace added.

> This commit prevents cfs_rq to be assgined new runtime if it has been
> throttled to avoid the above incorrect type cast.
> 
> Signed-off-by: Liangyan <liangyan.peng@linux.alibaba.com>
> Reviewed-by: Ben Segall <bsegall@google.com>
> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>