[PATCH 3.16 63/83] sched/fair: Don't assign runtime for throttled cfs_rq

[Ben Hutchings]

3.16.78-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Liangyan <liangyan.peng@xxxxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Reviewed-by: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: shanpeic@xxxxxxxxxxxxxxxxx
Cc: xlpang@xxxxxxxxxxxxxxxxx
Fixes: d3d9dc330236 ("sched: Throttle entities exceeding their allowed bandwidth")
Link: https://lkml.kernel.org/r/20190826121633.6538-1-liangyan.peng@xxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
[bwh: Backported to 3.16: Open-code SCHED_WARN_ON().]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3271,6 +3271,8 @@ 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
@@ -3450,6 +3452,11 @@ static u64 distribute_cfs_runtime(struct
 		if (!cfs_rq_throttled(cfs_rq))
 			goto next;
 
+		/* By the above check, this should never be true */
+#ifdef CONFIG_SCHED_DEBUG
+		WARN_ON_ONCE(cfs_rq->runtime_remaining > 0);
+#endif
+
 		runtime = -cfs_rq->runtime_remaining + 1;
 		if (runtime > remaining)
 			runtime = remaining;