[PATCH 5.2 08/37] sched/fair: Dont assign runtime for throttled cfs_rq

Greg Kroah-Hartman
2019-09-13T13:23:23Z

From: Liangyan <liangyan.peng@linux.alibaba.com>

commit 5e2d2cc2588bd3307ce3937acbc2ed03c830a861 upstream.

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, then other cfs_rqs
attached to this cfs_b 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;
}

The runtime here will change to a large number and consume all
cfs_b->runtime in this cfs_b period.

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 until that distribute_cfs_runtime is called.

Signed-off-by: Liangyan <liangyan.peng@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Ben Segall <bsegall@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: shanpeic@linux.alibaba.com
Cc: stable@vger.kernel.org
Cc: xlpang@linux.alibaba.com
Fixes: d3d9dc330236 ("sched: Throttle entities exceeding their allowed bandwidth")
Link: https://lkml.kernel.org/r/20190826121633.6538-1-liangyan.peng@linux.alibaba.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/sched/fair.c | 5 +++++
1 file changed, 5 insertions(+)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4449,6 +4449,8 @@ static void __account_cfs_rq_runtime(str
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
@@ -4652,6 +4654,9 @@ static u64 distribute_cfs_runtime(struct
if (!cfs_rq_throttled(cfs_rq))
goto next;

+ /* By the above check, this should never be true */
+ SCHED_WARN_ON(cfs_rq->runtime_remaining > 0);
+
runtime = -cfs_rq->runtime_remaining + 1;
if (runtime > remaining)
runtime = remaining;