[tip: sched/urgent] sched/fair: Don't assign runtime for throttled cfs_rq

[tip-bot2 for Liangyan]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     5e2d2cc2588bd3307ce3937acbc2ed03c830a861
Gitweb:        https://git.kernel.org/tip/5e2d2cc2588bd3307ce3937acbc2ed03c830a861
Author:        Liangyan <liangyan.peng@xxxxxxxxxxxxxxxxx>
AuthorDate:    Mon, 26 Aug 2019 20:16:33 +08:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Tue, 03 Sep 2019 08:55:07 +02:00

sched/fair: Don't assign runtime for throttled cfs_rq

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: stable@xxxxxxxxxxxxxxx
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>
---
 kernel/sched/fair.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bc9cfea..500f5db 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 */
+		SCHED_WARN_ON(cfs_rq->runtime_remaining > 0);
+
 		runtime = -cfs_rq->runtime_remaining + 1;
 		if (runtime > remaining)
 			runtime = remaining;